By the guidelines: 4,000 gp
Just so everyone’s clear, the guidelines suggest that a magic item that continually provides a spell’s benefit should cost Lspell × Lcaster × (2,000 gp) × C, where C is a constant depending on the spell’s regular duration. For a 1st-level spell at caster level 1st that lasts 1 minute/level like protection from evil, we have Lspell = 1, Lcaster = 1, and C = 2, for a total of 1 × 1 × (2,000 gp) × 2 = 4,000 gp.
By comparison to other items: ~30,000 gp
However, the guidelines are quite clear that you should only turn to the guidelines second—your first stop should be figuring out what comparable items are out there.
By the way, when I wrote this, I did not know about the phylactery of protection from evil that Prevarications notes in their answer—but it turns out that I came up with the same number that Paizo did.
A friend also points out the lawkeeper’s lock from Ghostwalk, which grants constant protection from chaos for 12,100 gp (and has a couple of one-off—as in, ever—effects). Ghostwalk is a very unusual campaign setting (all the PCs are “eidolons,” special more-playable ghosts), and its balance isn’t always great, either with respect to other books (which might be intentional), or internally (which is presumably a mistake). More about that price at the end of this answer, where I report that friend’s opinion.
AC and saves: ring of protection +2 + cloak of resistance +2
Protection from evil gives, among other things, a +2 deflection bonus to AC and a +2 resistance bonus to all saves—that’s equivalent to a ring of protection +2 and a cloak of resistance +2, which cost 8,000 gp and 4,000 gp, respectively. Per Magic Item Compendium, combining “generic” bonuses like these incurs no premium, so a combination item of the two would cost 12,000 gp—3× what our robe of protection from evil costs.
Which, in a sense, works out perfectly: protection from evil offers those benefits only against evil creatures. Assuming an even distribution of alignments—more on that in a bit—that means it only applies to ⅓ of all creatures. 3×⅓=1, after all, so that price arguably works out—at least for just the AC and save bonuses.
But of course, in reality, it’s impossible to really gauge what proportion of one’s opponents is going to be Evil. In a campaign where a bunch of holy warriors are marching into Hell for some rip and tear, it might be rather near 100%. In a campaign where the heroes are just generally-decent folks doing some exploring and “archaeology,” the majority of their foes might be rather-neutral monsters just looking for a meal. Maybe the heroes in the latter wouldn’t be very interested in protection from evil, but, and we’re getting ahead of ourselves here a little bit, there aren’t a lot of other options for the anti-possession and anti-compulsion effects.
Anyway, I’m going to call it ½—you probably don’t want to buy protection from evil if the proportion is less than that. That gives 6,000 gp—by comparison to the ring of protection +2 + cloak of resistance +2—for the AC and saving throw bonuses alone.
Anti-possession/compulsion: many not-quite-right comparisons
However, that was only part of what protection from evil does. It also protects against possession and compulsion—from any creature, not just evil ones—and protects against all non-good summons. Summoning isn’t that common, so the latter is difficult to evaluate, but the former is quite significant protection. Long-term, one would prefer mind blank, but since that costs over 100,000 gp,¹ there’s a pretty long stretch where protection from evil is the best protection available from possession and compulsion.
Irritatingly, that makes it difficult to price this component as there isn’t a direct comparison available.
There are a bunch of core items offering immunity to various subsets of nasty effects:
- periapt of health (disease) for 7,400 gp
- ring of mind shielding (mind reading and alignment discernment) for 8,000 gp
- necklace of adaptation (noxious gas) for 9,000 gp
- periapt of proof against poison (poison, natch) for 15,000 gp
At the high end, there’s ring of freedom of movement, 40,000 gp, but freedom of movement is ridiculous, immunity to an enormous collection of things, more comparable to the aforementioned mind blank than these others.
Ultimately, possession is a rare and niche danger, but compulsion is a serious threat. It’s not nearly as common as poison, but it’s almost impossible to overstate how much more dangerous it is, and it’s not rare enough to write off. If immunity to poison is 15,000 gp, immunity to ongoing compulsion is worth more than that. I’m leaning towards 20,000 gp, just for a nice, round number.
Hedging away summons: nothing comparable
There isn’t really any other effect remotely similar to protection from evil’s effects on non-good summoned monsters, and it’s really difficult to gauge how valuable it’s going to be. I would expect any Good adventurer to fight Good summons just about never (though of course it isn’t impossible), so the alignment thing is a very minor limitation. The bigger question is, how many summons are you going to run into, and how many of them are going to be stopped by it?
First, if you attack the summon, all bets are off. That makes this similar to sanctuary against some fraction of foes you face. Second, the summon is only prevented from touching you—if they use a weapon, ranged attacks, etc., then it does nothing. It also allows Spell Resistance to bypass it, which a lot of the more evil summons (fiends) have. Ironically, this probably most effective against druids, since summon nature’s ally has a lot of beasts that just want to eat you, and are neutral.
All told, I just don’t feel like summons are a major threat most of the time; they aren’t used by most enemies, and most things enemies can summon are fairly weak anyway. In a themed campaign against a lot of druids, it gets a lot better, but then in such a campaign the AC and saving throw bonuses get a lot worse.
I’m calling it 4,000 gp, just because I really don’t think it’s worth all that much most of the time, and it results in a nice round total. I don’t think it’s superior to the AC and save bonuses.
By cranking up caster level: 1,440,000 gp
Out of curiosity, I wondered what it would cost to have protection from evil with a caster level naturally high enough to last all day: turns out it’s a preposterous 1.44M gp.
By using Persist Spell: 7,000 gp or 91,000 gp
Persist Spell makes a spell last 24 hours, and is a +6 spell level adjustment. Thus a persistent protection from evil is a 7th-level spell. Tricky thing: is the minimum caster level for this still 1st, or is it 13th? There’s no good answer—the spellcasting rules never actually state that there even is a minimum caster level to cast spells, despite the fact that the magic item creation guidelines relies heavily on this concept. Metamagic, meanwhile, explicitly states that it doesn’t change the spell’s “actual” level, just the spell slot required—so even though a 7th-level spell “requires” (sort of) caster level 13th, it’s not at all clear that a 1st-level spell cast from a 7th-level spell slot does.
Anyway, you get either 7,000 gp or 91,000 gp. Both are poor answers to your question. It just crossed my mind as something that might be a workable approach, but it turned out not to be, at least here.
By practical consideration (read: my opinion): ~17,500 gp
I’m actually currently playing Age of Worms, and I wouldn’t have actually spent 30,000 gp on this. That’s the price that seems most consistent with what other magic items exist, but I don’t think it’s actually the right answer.
Here’s the thing: a lot of this cost is due to comparison to ring of protection +2 and periapt of proof against poison. Those are overpriced items that I wouldn’t recommend anyone buy.
More importantly, most people with this robe are still going to need a cloak of resistance +2. Unlike ring of protection, cloak of resistance has extremely competitive pricing. There are very, very few good reasons to not have the best cloak of resistance you can afford. Which means as soon as a cloak of resistance +3 becomes available, you’re going to want it even if you have continuous protection from evil. More importantly, you probably want the cloak of resistance +1 and cloak of resistance +2, too, because you probably aren’t facing 100% evil creatures.
(If you actually want to buy a ring of protection—which you shouldn’t—the same argument holds there, as well.)
At this point, pretty much the only thing of value left in protection from evil is the anti-compulsion effect. Which is definitely valuable! If medium fortitude (+3-equivalent) costs a minimum of 15,000 gp to add to a +1 armor, the anti-compulsion effect should cost about the same. Ongoing compulsions aren’t as common as critical hits, but the result could be even worse.
And since we can’t just give away the other effects, even if they’re worth way less than initially suggested, we’re looking at somewhere between 15,000 gp and 20,000 gp. I’ll split the difference and say 17,500 gp, though I suppose the anti-summoning thing deserves a little more than that.
By separating the effects: —/~15,000 gp/~8,000 gp
Part of the reason protection from evil is so difficult to price is because it does three fairly-different things. The AC and saving throw bonuses aren’t very useful to the overwhelming majority of adventurers, but potentially amazing for those who really only fight evil foes and aren’t going able to afford a cloak of resistance +3 anyway. The anti-summoning effects are very weird and difficult to price, but they’re probably best when fighting a bunch of druids—which is a very specific campaign that probably doesn’t overlap much with the one where everything you fight is evil. The anti-compulsion effect is the real value, but it’s sort of problematic that you also have to tack on the cost of the other very niche effects to get it.
If you offer them as separate items, you can just ditch the AC and saving throws altogether—just buy a ring of protection and cloak of resistance like everyone else—and you can safely price the anti-compulsion effects around 15,000 gp and the anti-summoning effects around 8,000 gp. I’m increasing the cost of the anti-summoning effect because if you’re buying it independently, I’m assuming you’re actually going to use it, as opposed to be it being “tacked on” to what you really want.
By level it becomes available: ~12,000 gp (a friend’s opinion)
The same friend who pointed out the 12.1k lawkeeper’s lock tends to agree with that price, with the following argumentation that I must admit, I find compelling.
WBL at level 9, when the more hardcore mind control and summon (demons/devils that summon more of themselves, mainly) effects start coming into play, is 36k
someone Worried About Those would find 1/3 of their WBL to be a reasonable purchase
the combined 25k proposal you've got would push that more to 12th level, which feels a little late
level 9 is where dominate effects come online (and more monsters get charm effects), level 9ish is around the time self-summoning devils and demons start coming online, and a devil summoning a buddy who shows up and immediately full attacks you is something that's gotten me before
It’s definitely true that 12th level feels late; that’s not far off from the 15th where mind blank becomes trivially available. And considering when you’d expect such an effect to be available—and how much of one’s wealth should be dedicated to it—is really just about the best way to evaluate custom effects, because ultimately that’s what matters. Deciding “a third of a 9th-level character’s wealth” is pretty reasonable.
- Ring of mental fortitude, 110,000 gp in Dungeon Master’s Guide II; third eye conceal, 120,000 gp in Expanded Psionics Handbook; the guidelines also suggest 120,000 gp. Note the guidelines also allow a 1/day command word item of mind blank for 43,200 gp, though I suspect that many DMs would—and probably should—balk at that.