How much

[Nosta]

would a bracer that grants a +10 to preform cause?

[Xervous]

would a bracer that grants a +10 to preform cause?

Going off the +10 Ring of jumping, 10,000gp

[John Longarrow]

It would be 10k unless it was limited some how. If specific to one subskill it may be at little as 7,500.

[Curmudgeon]

There is no Perform skill.
Perform (Cha)

Like Craft, Knowledge, and Profession, Perform is actually a number of separate skills.

You could have several Perform skills, each with its own ranks, each purchased as a separate skill. If you want an item that will improve every Perform skill, expect to pay Epic prices.

[AvatarVecna]

There is no Perform skill. If you want an item that will improve every Perform skill, expect to pay Epic prices.

Sure, +10 might be epic if you tried to make it by RAW, but a +3 is totally serviceable pre-epic.

[Uncle Pine]

Custom magic items that grant a competence bonus to a single skill follow the [bonus squared x 100 gp] formula. So a +10 Perform (one) bracer would cost 10,000 gp. More informations here (http://www.d20srd.org/srd/magicItems/creatingMagicItems.htm).

And Curmudgeon is right remembering you that you need to choose what Perform skill the item enhances, or pay for multiple Perform skills.

[tiercel]

Complete Adventurer has the Choker of Eloquence: 6000gp for the lesser version (+5 Perform/Sing, Diplomacy, Bluff) or 24000gp for the greater version (+10 to same skills).

It's possible you might actually prefer this item; it is consistent with the standard pricing mentioned by others in this thread (10000gp for +10 to a specific Perform skill, sans cheese).

[Chronos]

Actually, there is a non-epic item that grants a bonus to all Perform skills: The Circlet of Persuasion (http://www.d20srd.org/srd/magicItems/wondrousItems.htm#circletofPersuasion), which also provides the same bonus to Diplomacy, Bluff, Use Magic Device, etc. It's only a +3, but it only costs five times as much as an item that provides the same bonus to any one skill. If we take that as precedent, then a +10 circlet of persuasion would cost 50,000 GP.

Also note that magic item prices have an additional cost modifier for being on an inappropriate body slot. Bracers might be an appropriate slot for, say, String Instruments, but they're probably not appropriate for Singing. Which Perform skill in particular were you interested in?