If I'm doing something for free, I don't mind doing it for free (obviously). If I'm doing something for money, I want to be paid my going rate; otherwise, either find someone else or sell me on just volunteering. I'd never charge $20/hr for software stuff -- it's basically $0/hr (which is most of my time, by choice) or market rate. It's actually the market rate billing that free me up to volunteer with open source projects in the first place.
Studies back you up and suggest "Pay enough or not at all." Paying below market rate tends to just piss people off and make them feel insulted and undervalued.
