For a spellcaster, you want Mithral light armor, usually. a Mithril Shirt (DMG, magic armor section; costs 1100gp, canbe furtehr enchanted -- the 1100gp price is for a NON-magical mithril shirt).
From Magic of Faerun, "Livign Metal" construction is also an option. After wearing it for a week, it has finished fitting itself exactly to the wearer. The Dex Bonus allowed is raised by one, the Armor Check is reduced by 1, and Arcane Spell Failure is lowered by 5%. Additionally, Living Metal has HArdness 12, 30hp/inch, and, regenerates/heals/self-repairs 1hp/minute (be VERY AFRAID of a living metal golem, lol).
All this for 700gp (light armor), 2000gp (medium armor), or 4500gp (heavy armor).
Generally, Mithril is the best bet, aside from the self-regenerative properties (and sheer niftiness factor) of Living Metal.
As for a custom enhancement, "Spellcaster's Armor" ... I'd suggest perhaps a 10%, 20%, and 30%, with prices comparable to Fortification's Light, Medium, and Heavy levels. Call them Lesser Spellcaster's, Spellcaster's, and Greater Spellcaster's enhancements. As a thought.
