I have never charged anyone to make a quilt, so I don't know how to help there....however, the suggestion of having the person pay for the fabric is a good one. I have made quilts for friends, my only requirements were that they select a pattern from one I was comfortable making AND they select and buy the fabric, batting, and backing. In each case, my friends were amazed at how much it cost just for the fabric and batting. A few offered to pay me for my time, but I told them it would take at least 40 hours and they couldn't afford me. It did open their eyes to what a quilt would cost. They had no idea.
I suspect not many people really understand what a quilt should cost, but having them price fabric, batting, etc. would let you know how serious they are about the idea. ( My son was just here in my sewing room and asked how things were in the quilting world. I told him what we were talking about, and he said he thought most people would change their minds once they learned what making a quilt would cost. I wonder if he is right? I know that I warned my friends that even just buying the fabric would cost more than they expected, that quilting was not a cheap hobby...)
Dina
Dina