This has been discussed many times, and it depends on a lot of factors. The Etsy suggestion is a good one. Although you are considering parting with a completed quilt, remember that if you do decide to take a commission, get everything in writing and a 50% deposit to cover your outlay!
Here's a good discussion with a link to the worksheet they talk about
:
http://www.quiltingboard.com/main-f1...t-t262731.html
http://mollisparkles.com/2014/03/tgiff-costing-sheets/