I don't have a hard-and-fast formula either. I usually charge at least double what the fabrics cost (at current quilt shop prices) plus batting, thread, etc. I also factor in marking the quilt, the difficulty of the piecing/applique/quilting and whether I had to design the quilt myself (I do a lot of custom work) or had a established pattern to go by. I machine piece and applique, and quilt on a hand-guided machine quilting frame, so I also charge a little to load the quilt into the frame. I usually ask for 50% of the estimated cost of the quilt up front, so I'm not personally out that money during the time it takes to make the quilt. The best advice I can give you is to not cheat yourself, your time and talent is too valuable!