I recommend the economic theories explored in this series (read the economicon first). In their prior works of Tome of Necromancy and Tome of Fiends*, they explored the ramifications of what amounts to XP as currency in the "liquid pain" and "ambrosia" sense. (BoVD, BoED) Concentration (the neutral) version is described in the Book of Gears which is very well written and highly recommended. You can find K's other writings from that link.
Short version. If you don't want to revamp the 3 economies of D&D 3.5, just make 1 XP cost 4 gold (and, if possible, make that gold instantiated in magical monster organs that require quests.)
- Giving people the "Or Giant Frog" option for chaotic characters for quite a few years now...
In this model we get a coherent explanation for why, when all the forces of Evil are composed of a multitude of strange nightmarish creatures, and the forces of Good have everything from a glowing patch of light to a winged snake tailed woman, every single soldier in the army of Chaos is a giant frog. This is because in this model Limbo is a place that is totally insane. It's a place where the answer to every question really is "Giant Frog". Creatures of Chaos then proceed to go to non Chaotically-aligned planes and are disappointed and confused when doors have to be pushed and pulled to open and entrance cannot be achieved by "Giant Frog".