This question is really interesting to me. I have made a deal with a music producer who later became a close friend of mine. We agreed on 7% of total income from sold games.
We found this fair because that way if we both put a large amount of work into a project but no copies are sold, then I'm not X amount of dollars out of pocket. Just his time and mine.
But then conversely if the game in question made millions (pfft, I wish) then we would both get a very decent lump sum. We also took into account the difference in hours worked on a project. He would work on the project about 1 hours to my 10, roughly - so pure time accountancy the rate seemed fair also.
If you're sourcing your stuff from a commercial website, then you're most likely looking at a single payment for the rights of a specific track. About $600-$1500 is probably reasonable. A lot of licenses also scale up. So something like $300 for a game with 3000 copies distributed but $1000 for a game with 100,000 copies distributed.
It depends on the size of your market and your budget, as well as your source.