A professional composer who is not willing to work "for cheap" would charge at least $200 per minute of completed music. That might seem like a lot but many hours can go into producing just one minute of music, if it is high-quality. If I am fast and make a track in 4 hours, then do two revisions each taking about an hour, I would be making $33 per hour. That's pretty good... but a really high quality composition will probably take more time than that, and there's also the time and money spent on acquiring and maintaining equipment, software, etc.

Of course, in the indie world, budgets are slim and many people work for less than the "industry standard," sometimes for free just to improve/gain exposure, sometimes for the promise of revenue share down the road. So unless your project is expecting to get a ton of funding, it might be more realistic to plan that you will not be able to pay as much up front. Maybe somewhere around $80-$100 per minute. That might be acceptable to an decent indie composer, especially with rev share on top of it.

There are other variables too: How much music do you need and how complex should it be? I can bust out simple chiptunes with 4-5 instruments all day long, so I would expect to get paid less for that. But a major orchestral composition requires tons of instruments... you could be looking at 20 or 30 individual tracks, and suddenly the time investment is much bigger.

So I will just guess to fill in those blanks. Let's say you need 30 minutes worth of soundtrack for your game. You want it to be good quality, but don't need AAA-level orchestral masterpieces, so you offer $100 per minute plus a small percentage of revenue. That's going to be $3000 up-front. As the person hiring for that job, I would keep in mind that that's pretty cheap. If the game doesn't make much revenue, the composer will have done all this work for around $10-$15 per hour. So I wouldn't expect the most professional music in the world or push them too hard to do revision after revision.

All of this can be very different for your project. Composers can also be paid hourly, although this can cause tension if they are taking longer than you think they should or if you think they are lying about their hours. Charging per minute of music means you know how much you are spending, and the composer can decide if the time investment is worth it. If you are not able to pay very much, you'll have to make concessions on the quality and complexity of the soundtrack, and/or be willing to "sweeten the deal" with things like revenue sharing.