If you got a fair price for the one pictured and were satisfied with it. I would take that price and times it by how much bigger this one is. Is it twice, three times or four times larger than the first one.
As a buyer, I think that she would accept the logic behind the price. However- if you gave her a great price on the first one, just let her know that you can't do it again due to the amount of time involved.
Good luck...Love the quilt!