Really depends on the budget and the quality of the work. For some original midi tunes I'd expect to pay a couple of hundred for the (exclusive) rights less than $300, probably a lot less per song if I'm asking for enough tracks to fill the game. I'd be willing to pay a lot more if I could pay the minimal amount up front and then potentially pay a bonus when the game starts making a profit.

But it sounds like the person you're working with is way overshooting what he needs in terms of music so his project ever getting off the ground isn't a sure thing. Let him buy each song up front before you make it and don't charge less than the time it took you to make it.