I've become a bit of a go-to guy for music videos in Nairobi with my current policy on payment: If I'm learning something about filmmaking, that comes OUT of my fee, maybe even completely eliminates it. If I have never shot a music video before, I sure as hell shouldn't charge someone for a product that may or may not be complete shit. If I've graduated and I now have an opportunity to use better cameras, lights, dollies, and a crew, then I can start charging for lesser gigs, where I don't GAIN from the experience myself, but rather GIVE. It's simple supply and demand. If the demand comes from both ends, then I consider that a TRADE rather than a PURCHASE, per say.
So if you're in "learning territory," really getting good experience you wouldn't get elsewhere, then give yourself and the band a deal: do it for free or next-to-nothing. You won't (shouldn't) get too many complaints if you don't do a perfect job in the end, and both people gain valuable experience. And if you actually do manage to do a great job, then you have an impressed client liable to recommend/use you again, plus all that valuable experience!
Then when you're doing more complex shoots that demand more technical know-how and leadership skills, you should definitely start charging for jobs that don't teach you anything about cinematography, or offer anything back to you.
My two cents.