My Rates:

1.  Pay per track basis: $30 per completed, approved non-orchestral track (jazz, electronic, rock, light orchestration). And $50 per completed, approved heavy orchestral track (full orchestral, lots of instruments).  I've used this rate with many indie game devs.  So, you can pay one track at a time. 

or

2. A one time, flat rate fee ranging from $200-$300 (depending on budget, level of orchestration, amount of music, etc.).  On most indie films, I've worked on a flat rate fee basis. 

So, I'll work on either a pay per track basis ($30-$50 per track) or one time flat rate fee depending on budget.