That's a hard one for me...was this family close to yours? If so, I would probably just make it as a gift of love for the loss of their child. If it is a person you don't really know and were asked to do this then I would charge for the batting and backing a a small fee for making it. I'm not one to charge for my quiltmaking because I love it so much and am thrilled when someone wants me to make one for them for whatever reason. But with fabric prices as they are it is not unreasonable to ask them to pay for what you would need to purchase to complete the quilt and an amount for your time.
You will receive many responses here about people that do sell their quilts for specific per inch amounts and that is fine and also reasonable. For many people it is an additional income. I have just been fortunate in not having to do that and love to pass along a gift of love to someone. That said, it is only very special people, donation quilts or family that I do my quilting for so that's probably why I feel the way I do.
Don't know if this truly helps you out but just my very humble opinion.