I suggest you do a search online and in a local store if you have any and see what the going rate is in your area for a similar size and content quilt.
Doubling the price usually refers to small projects of quilt or craft.
Big quilts are differant because they have so much more work on them.
Visit with a quilt shop owner and see what they think is appropriate for your area.
Prices can vary depending on the availability of such size quilts depending on the area.
I sell to an antiques and gift shop in a local hight traffic tourist area and do quite well there but I do not do well where I live. So it depends on the market as to what is fair!
It also depends on what the person is willing to pay!