I've given some thought to Materia, too, though unfortunately I've never put any of it in writing. Here's my thoughts, in a nutshell:
-Magic materia: priced as a 1/day command word item, and can be used as such.
-Spellcasters can cast a spell from a materia they have equipped by powering it with an equal or greater level spell slot, provided that spell is on their class list.
-Independant materia/command materia: like any normal buffing wondrous item.
-All costs multiplied by, say, 1.5x over the normal item pricing guidelines.. slots are limited, but not so much so as conventional body slots.
-Any item can have any number of slots for a relatively small cost per slot (limited by size, perhaps).
-Materia with multiple spells: priced using the usual guidelines for multiple abilities in a slotless item.
-Support materia: pricing would be similar to the metamagic rods, and for the sake of simplicity it would always apply its metamagic effect to the linked materia. More esoteric ones would have unique descriptions - Elemental linked to a magic materia with an elemental descriptor would add that property to a weapon, or add that resistance to armor.
I'm sure there was more, but that's all that comes to mind right now.
And for Sollir: Mime or Calculator, please?
--Impeesa--