I'm a costume maker myself, and I have this rule- with friends if I'm doing a performance with you, you pay for materials and the costumes are still owned by me. If I have nothing to do with the group then it's full commission rates with a contract.

I learnt this the hard way with friends. A combination of friends wanting things and me not saying no led to me taking on far too much work and things then getting ugly when I couldn't do it all. I sorted that out but it did burn a friendship I'm only now repairing. Business is business even if it's a friend commissioning you!