Music can add an incredible amount to a game if it is done well.
Approaching this from a developer perspective, I think you're going to run into a few issues on this one that you'll need to address and/or overcome to make it work:
- Some indie devs will have some pretty unrealistic expectations as to what others will work for, as well as the potential degree of success of their game. You'll need to be prepared to cull these ones.
- Some indie devs are more realistic, but simply won't have the budget to get original soundtracks, and will be looking for non-exclusive tracks. It might be easier for example to persuade twenty devs to pitch in $25 per completed minute for limited exclusivity than one to pay $500/min for full exclusivity.
- The ones that can pay will either want to see (hear!) some of your past work or know someone that you've worked with previously before they fork out anything resembling a fair payment. If your brother doesn't have a portfolio that potential clients can browse, and his competitors asking for less do, he's going to have trouble finding clients willing to pay that much. If he's got all of that, it'll come down to what others are asking for, and if his work is of similar standard.
What will people pay? They'll look around at what is out there, and what they'll get for their money. They'll try to balance the best price against the quality they are likely to get, within their budget constraints. Your brother then needs to make sure that they find him.
All the best to you both.