How do I balance Elder Scrolls soul gems with the loot progression?

Question

I'm currently running a Pathfinder game set in Tamriel, the setting of the Elder Scrolls games. A couple of my players are very interested in using soul gems and enchanting. My first instinct was to treat soul gems like Eberron treats dragonshards; they're just a part of the materials required for magic item construction. However, I'd really like to be able to let players Soul Trap creatures and harvest their own soul gems. I've designed a system to allow this, below:

A Soul Gem is an item that weighs 1 ounce, and costs 10 septims. There is only 1 size of blank soul gem. Souls of any size can fit into a soul gem. A soul is worth an amount depending on the HD of the creature, according to this table:

HD  Soul Value in septims
1   75
2   300
3   675
4   1200
5   1875
6   2700
7   3675
8   4800
9   6075
10  7500
11  9075
12  10800
13  12675
14  14700
15  16875
16  19200
17  21675
18  24300
19  27075
20  30000

For creatures with HD over 2, the formula is 75*x^2 = y, where X is the HD of the creature, and Y is the septim value of the soul. When creating a magic item, 75% of the materials cost is in soul gems. For example, an item that costs 1000 septims market value requires 375 septims worth of souls.
Soul gems can be partially discharged when making magic items. When a soul gem is totally emptied, it crumbles to dust. Soul Trap can only be used on blank soul gems.
If you use only one fully charged soul gem in the creation of a magic item, and you completely deplete the soul gem in the process, you may add 2 to your effective caster level for the creation of that item, and add 1 to any save DCs the item provokes. For example, if you used a 1 HD soul gem (worth 75 septims) to create a scroll of a level 1 spell, and you were level 2, you could create a scroll with a caster level of 4. You still need to pay the materials cost for the item (13 septims for a CL 4 scroll of a level 1 spell). This may allow you to create items that you do not meet the caster level prerequisites for.

Soul Trap is a new spell added to every class list except Paladin and Ranger:

Soul Trap
Mysticism
Level 1
Casting time: 1 standard action
Components: V,S,F (A blank soul gem)
Range: Close
Target: One creature
Duration: 1 round per level
Save: None
Spell Resistance: Yes
If the affected creature dies while under the effects of Soul Trap, 
their soul is trapped in the soul gem used as the focus.

Does this system seem balanced? How do I fit this system into the standard loot progression?

Answer 1

I like the idea, but it's too rich.

You're basically saying a 1 HD creature is worth 75 gp equivalent value (as it's one to one to gp when used to make magic items), so it needs to give 75 gp less in cash money. Refer to the Pathfinder treasure value per encounter guidelines.

Let's compare against the example they use of an EL10 encounter with an 8 HD stone giant and 4 4 HD gargoyles. If you're using the medium pace that encounter should be worth 4250 gp. It looks like using your rules the PCs can soul-trap 4800 + 1200 + 1200 + 1200 + 1200 = 9600 septims, which is way high. I might discount it a little because of the need to cast the spell, but especially as the spell has no save I'd personally bring along a level 1 goon with a bunch of scrolls I've scribed for him to get the sweet, sweet free money. Even if they just trap the giant, that's more than the gp value that the encounter should provide - and that's with zero treasure over and above the soul.

Because face it, magic is the #1 use of money in games after a certain level - so I wouldn't discount it all that much.

What you need to do is match up "soul value" those charts in the PRD so you're not giving a huge windfall.