First off, to the OP: what edition or system are you playing?
This will make a big difference.
1e already has a system (but from your post I assume you are not playing 1e) that is easy to tinker with if you don't like it as is.
2e can work the same as 1e if you like, or again you can easily streamline it or tinker with it.
3.xe can be made to work, I've seen it done; using a relatively simple system where it cost about 500-1000 g.p. per level being trained into and usually took a week or two. (that said, the advancement in that game was slowed down; I'm not sure if it'd work so well were advancement kept at by-the-book speed) There's also the UA system someone already mentioned.
4e I'm not so sure about. If you're using 4e's canned adventures, or running things close to by-the-book, 4e tends not to give out enough extra treasure for this to work all that well - unless you want your characters to be nigh-broke all the time - and also keep in mind they're sometimes supposed to bump several times in the same adventure for the adventure to "work". KotS, for example, assumes they'll be 1st level to start and 3rd by the final few encounters.
Don't get me wrong. I'm a big fan of training times and of there being some expense involved; but to make it work well in either 3e or 4e you're probably going to have to make some other changes as well, the most notable of which will probably be slowing down the advancement rate and - if you're running 4e - increasing the treasure haul a bit.
Another thing to consider in terms of tweaking whatever training system you use: will untrained characters (i.e. they have bumped but not yet trained) be able to gain any ExP. I'd vote for yes, even if it's only half-rate, if only because sometimes it's impossible to get out of the dungeon to train and the game probably needs a mechanism to accommodate that.
Another consideration: at higher levels (say 9+ in 1-2e, 12+ in 3e, 15+ in 4e), you might want the PCs to be able to self-train rather than have to find a trainer; either that, or you're going to have to introduce a set of uber-NPCs that can serve as high-level trainers; and that can cause its own headaches if you're not careful.
One huge advantage of enforced training is that it drags the PCs out of the field for a while: the world has time to advance while they're off training, and it makes the 1-20 (or 1-30, whatever) progression take just a little longer in game time.
Lanefan