Do I want to work on free projects? That's a tough one. I've tried over the past two months to work with some hobby developers for free, naturally, because to be honest, my portfolio of actual games shipped is currently zero. Teams have fallen apart. Developers have stopped responding after I give them music, so I'm left in the dark as for when/if the game will be finished. Eventually, I'd like to get paid for my work. But its hard to get paid when you don't have work to show, the whole chicken/egg thing. So, for now, I guess I'm working for free. But, my conditions for free are: a strong GDD, a history of shipping work, rev-share if more than say... 4 tunes.
As far as pricing goes. For fully produced and polished tunes I'd charge $50 per minute of music for exclusive rights, with up to 3 revisions. I can usually finish a 3 minute tune in about a day or two depending on the complexity required. All of the tracks in my portfolio, except the dubstep ones, were made in two days or less.