Well, I was browsing around on the plugins and bukkitdev list when I found that one very simple idea just hasn't been created- Buying experience using economy. Something like one level for $100 would work just fine. Don't think it'd be too tricky to code but I do feel as if it should be there. If a plugin like this already exists and I just couldn't find it because I'm a derp, please link me to it and I'm sorry for wasting your time.
You'll be able to with admin shops and whatnot via ChestShop once 1.2 releases, since EXP can now be given via a creative mode item. Of course, until Jeb makes it craftable, ordinary users will not be able to get them without admin help. So I like this idea.