I think it's a hard question to answer. I'm working on a quilt for someone right now and I always have a hard time coming up with a figure. So now I do an estimate and give that to the person to decide if they want it done or not. I figure the cost of materials and then the number of hours of labor. I charge $10 an hour. I also give a range on both, then give the lowest and highest costs to them so they will have an idea of what I'm basing my total on. For the t-shirt quilt I'm doing now I estimated between $250 and 350. The quilt will measure 78 X 85 and is home machine quilted.
Good luck with your project.