You can politely tell her that you could sell it for the cost of materials only, but not for less. Then figure fabrics - easy way is full size of top = how many yards = $6 per yard (I am thinking used a 40% coupon) double this because you have a top and backing. Add in cost of the batting. Personally, I would then add time and work, but, if you are trying to be polite, this would be an absolute base cost. Chances are she will not want to pay it anyway! Someone who would offer $20 for a handmade quilt has no idea of the work you did.