Well, there are a LOT of parameters, a lot of it depends on the difficulty of the quilt, your relationship with the person, how much fabric it will take, how much time it will take, your level of experience, etc.
I had a casual acquaintance ask me once to copy a quilt that her son had owned for over 20 years. We could not find a pattern for it. I gave her a written estimate that included time AND materials: how long it would take me to draw up a pattern, how much fabric it would require, and how much time it would take me for each step; cutting applique, cutting borders, stitching the applique, quilting, etc. She wasn't happy with it because she assumed I would simply use fabric from my stash and not charge her for drawing up the pattern. Ha - I don't THINK so.
Anyway - I would charge the true cost of every material used, including batting, backing, and binding, and very carefully take into account how much time you will invest in this project, and charge an hourly rate commiserate with your experience.