I was just wondering, for items in Pathfinder 1e that provide saves bonuses (like a Cloak of Resistance), they seem to only go up to +5, but in the Magic Item Creation rules, it looks like you could go much higher, all you'd need to do would be to calculate the different cost (and have that much gold to begin with, of course). Is a save bonus higher than +5 on any one item legal, or is there a hard limit to that?
Ultimately, there aren’t “rules” for creating custom magic items—there are guidelines, and those guidelines are for the GM to help them decide what to allow and at what value. So nothing based on those guidelines is strictly “legal,” and your GM can (and should, per those same guidelines) not allow some of the things the guidelines suggests. They’re not a perfect tool and they explicitly warn GMs that they still have to use their judgment on individual cases.
That being the case, the only really “legal” items are those that were actually printed in the book. And the best cloak of resistance that’s seen print is the cloak of resistance +5. You can definitely make that, because it’s an item in the book. (The GM can, of course, always ban an item as a houserule—but that’d be a pretty weird choice in the case of an item as basic and fundamental as a cloak of resistance.) The guidelines say a resistance bonus to saving throws is worth bonus² × 1,000 gp, which the cloak of resistance +5 matches exactly by being worth 25,000 gp. A hypothetical cloak of resistance +6 would thus be worth 36,000 gp. But since there isn’t any cloak of resistance +6 in print, it’s up to your GM whether or not that item is one that is possible to craft.
In my experience, most GMs do not allow a cloak of resistance +6.
Edit: I now realize my mistake in lumping wondrous items with magic arms and armor but I'd like to leave it here as an explanation for why cost tables exist to +10 even though item enchantment bonuses are limited to +5. That info could be helpful to those landing on this question in the future.
This is actually something I've run into lately myself. I found this quote sourced from d20pfsrd Magic Armor
... Magic Armor bonuses are enhancement bonuses, never rise above +5, and stack with regular armor bonuses (and with shield and magic shield enhancement bonuses). ...
In addition to an enhancement bonus, armor may have special abilities. Special abilities usually count as additional bonuses for determining the market value of an item, but do not improve AC. A suit of armor cannot have an effective bonus (enhancement plus special ability bonus equivalents, including those from character abilities and spells) higher than +10. A suit of armor with a special ability must also have at least a +1 enhancement bonus.
This effectively means that the enchantment bonus of the item is limited to +5 but you can go up to +10 in cost by adding abilities such as Billowing or Spell Resistance that don't effect the enchantment bonus. That's why the cost table goes up to +10.
I know that the Cloak of Resistance from the question isn't "armor" in the sense that it goes in the armor slot and adds AC, but I found very similarly worded entry on d20pfsrd Magic Weapons so it's probably safe to assume this applies to all magic items.
... Magic weapons have enhancement bonuses ranging from +1 to +5...
... Some magic weapons have special abilities. Special abilities count as additional bonuses for determining the market value of the item, but do not modify attack or damage bonuses (except where specifically noted). A single weapon cannot have a modified bonus (enhancement bonus plus special ability bonus equivalents, including those from character abilities and spells) higher than +10. A weapon with a special ability must also have at least a +1 enhancement bonus.
So in the end, you can have your Cloak of Resistance +5 with Spell Resistance, but not a Cloak of Resistance +6.