This comes up all the time. I have heard the 3x cost of materials before, and I do believe that some people use that as a guideline, but to me it doesn't make sense. The work in making a quilt is the same whether I use $4 fabric from my stash or $12 fabric new from the quilt store. Should I charge $12 for making one quilt, and $36 for the other?
If you want to make the quilt, then come up with a figure that you think is fair to you. It is up to the buyer to determine whether that figure is acceptable to her. Don't be surprised if the buyer suddenly realizes that the quilt will cost more than she is able or willing to pay.