Call of Cthulhu d20 has a Sanity point loss and/or (temp.) ability drain cost for spellcasting. How would it be best to introduce this system into DnD3.x?
Are there (semi-)official guidelines for this? If not, how would you do it? Would you introduce Sanity as well, or replace it with something else (hit points, for example)? Would you set up a general, level based cost for spells, modified by school and/or the DM's whim (...I mean, based on actual circumstances, spell effect, spell target etc)? How would you rebalance the spellcasters' power level compared to the other classes? Would the cost system apply to divine spells as well?
Seems like there are quite a number of sub-questions - but they're just that, sub-questions (to be carefully considered) of the main one, which is, once again:
How would you introduce the spell costs of Call of Cthulhu d20 into DnD3.x?
Bonus points for considering the adaptation to Ravenloft 3.5's rules as well (as an addendum to the generic DnD3.x ones.) ;)
Important edit: Please, do not recommend using a different system (even if it has similar effects) only. Feel free to mention such, of course, if you think it's better than what I'm considering, but do focus on and include your recommendation for the CoC d20 + DnD3.x rules fusion. Thank you. :)