I would rather pu the 10% at higher level, +2, or maybe even +3. Spell failure is the only way to prevent a wizard or a sorcerer to not wear an armor, so I would expect an armor without spell failure to cost around the same as the bracer of armor of the same value.
A mithral chainshirt +1 of lesser casting would cost only 5100gp to offer the same protection of a bracer of armor +5, and that, without spell failure or any other kind of penalty.
A mithral Breastplate +1 of greater casting would cost only 29 200gp to offer the same protection of a bracer of armor +6 (36 000 gp), and that, without spell failure or any other kind of penalty. A bit more reasonnable, but following that logic, I think it would be better to have that:
Casting, Lesser: reduce the arcane spell failure chance by 10%.
Faint Transmutation; CL 7th; Craft Magic Arms and Armor, haste; Price +3 bonus.
Casting: reduce the arcane spell failure chance by 20%.
Faint Transmutation; CL 9th; Craft Magic Arms and Armor, haste; Price +5 bonus.
Now, if we want to offer to our spellcaster cheaper AC armor bonus, that's another thing, but I don't like much that idea. For higher cost, I don't mind. And it would worth the cost, as you can put other enchantment on your armor that the bracer cannot have (like Shadow or Fire Protection) and free the bracer slot (if you want to do an arcane archer with greater barcer of archery).