Double price of bracers of armor for a constant effect.
I'm sure the magic item compendium would have something to say for 'odd-ball' effects such as dropping spells to activate.

If you do go 'dropped spells' I'd recommend an armor bonus to AC equal to the level of the spell level dropped, functions for 1/round per CL of the creator, and is an intangible force armor.
I'd say base price should be about...?
Use as much as you like, just drop the spell first.
24.000 Gp.
([16,000 for Bracers of Armor +4]x1.5 for odd-ball effect, and slot-less item.)