Its always difficult to know what to charge a relative or friend, but I have to agree with the last post seems a bit low. Yep your right no one realises just how much work goes into a quilt. Also are you hand or machine quilting. Could you not ask or google a person who does that type of thing and email the question how much they would charge, then you might get a fairer idea of how much it would cost.
Maybe some of the others might have ideas on this one Good luck
Elle