So, I'm about to drop Thoon Mind Flayers from MMV into my game and I want to make quintessence intriguing to the players, so I'm thinking about allowing them to use it as a replacement for XP requirements for item creation and spells/powers (with a gallon equaling X experience points) but there are two catches:
1. If they replace enough XP in spells/powers/item cration in the entire life of their character to equal half of the XP needed to get from the start of their current level to the next level, they'll have to pick either an illithid haritage feat from CPsi or an aberrant feat from Lords of Madness the next time they get a feat (even if it's a fighter/mage/psion bonus feat) to account for the corruption that this causes.
2. I'm not telling them about catch number 1, though I will strongly hint at it with things like "it worked, but it felt alien and wrong"
What I'd like to know is what you think would be a good XP/gallon ratio and what would be a good market price (bear in mind they may be finding a lot of the stuff and some magic item makers they try to have use it will refuse to use it again do to the "wrongness" the felt while using it). Also, if you have a different way to make the stuff enticing but altogether detrimental or a simpler way to show the corruption it causes or something similar, I'd love to hear it.