You should charge them (n-1)/n of what you would charge another band, where n is the number of people in your band. You might try asking them what your engineering contribution is worth. If they are cool they will take this question seriously. You probably put a lot of hours into it and that contribution should be recognized. You've been doing for free which makes sense while you are learning. Now, maybe less sense. As you gain other paying clients you will have less time for your band's projects. How will you feel about prioritizing by what your clients are willing to pay?

P.S. I have no idea what I'm talking about. Good luck with your decision.