There are so many variables! If you were quilting as a business, you would have to base your decision on making a profit on materials and getting a decent hourly wage for your time. On the other hand, if you're going to finish the top anyway, but have no use for it, and the money would come in handy, then charge whatever you would be happy to receive. Don't be surprised if the person interested in the quilt knows nothing about the cost of fabric, or the number of hours required to make a quilt (especially with hand quilting).