The first thing I would ask myself is "Do I really really want to do it?" I have gotten myself into fixes before because I wanted to please my family and not disappoint anyone. Because I made one afghan as a gift, I felt that I had to make one for everyone. I had so many to make in a limited amount of time, that it turned into a full time job - day and sometimes all night. So please be sure you want to start making quilts for charge for friends. One may lead to another and another and so on.
On the other hand if you are sure you want to do it, try to set a price that takes everything into account as Sally and ckcowl said. Then if it comes up again with someone else, you will be able to be equitable with everyone. Good Luck and remember to have fun with it. :)