In the DMG2, the is a Fighter's School or something like that in the example town of Saltmarsh...it gives a figure for how many XPs your PC earns per semester or something like that, and how much said semester costs.
Not sure that helps, but it may be worth looking up for ideas?
Oh, hey - Wish! 25,000 gp for 5,000 xp.
Oh, hey - I can turn that 25,000 gp into 25,000 xp. I just leveled up! And, oh, hey - I've got 5,000 excess XP.....
Interesting idea. I wouldn't be opposed to adopting this house rule, but I would definately have to put a cap on it somehow. Something along the lines of "you cannot buy more than 10% of your next level-up" or something.
I think DMG2 had somthing like this in the Saltmarsh description. It had rules for joining a combat school which granted exp for a CR six encounter at certain intervals. I would perefer that to a straigh conversion rate because it would mean that over time you would need to find new schools, a quest in itself, after 'growing' out of old ones.