think about how much you would work at any job for--and then make sure that you get at least that much plus your materials cost out of the project. I made the mistake of WAY underestimating the amount of time that it would take to "deconstruct" shirts and pants for 3 memory quilts project I'm trying to wrap up--and consequently have cheated myself.
Forgot the "rule" to estimate your time--then pad it with some hours for the problems you run into.