You'd think that fixing a Wizard through using an XP tax would be a simple task. The concept is simple: similar XP should yield similar power. Therefore, all the Tier 2 and Tier 1 classes should cost more XP than the Tier 3 classes. Yet, how do you actually price that correctly, especially when you run into issues of runaway power? If I do this right, I balance the wizard out, but if I do it wrong, I've only created a hassle.
My solution is this: all spells cost experience to cast, the XP cost of which is exactly the same as if the spell had been scribed onto a scroll.
So yeah, the wizard can go crazy casting all the spells that he wants to, if he really wants to, nickle and diming himself. Or maybe he'll want to hold onto his XP, restraining himself voluntarily, just like wizards in the stories do.
This should have the side-effect that wizards will gear themselves to be extra-powerful when they do use their magic, which is good, as a wizard opening up should feel like a nova.
If a wizard doesn't want to attack, he can use his "tactical advice" to give his attack to someone else at a +2 to hit.
Will that work? I have no idea, but it beats the tar out of any other crazy pricing scheme that I can think of.