I would keep track of everything you had to buy for that particular quilt and then charge 4 times what you paid for materials. That, then, includes your time and labor. Be sure to keep track of the quilting, either hand, machine or tying.
My sister was a clothes designer and said that big companies charged 4X the amount of labor, materials and time. I did the same when I was in the ceramic field. You are not making a great fortune, but if you have supplies left over, you can count that again for your next project.
If it costs you $200 to make a double quilt you charge $800 for it.
Sounds steep, but no one ever understands the time and effort you put into it. I am in the process of making a quilt right now for my husband and I and I have been working on it since last June. Oh, yes, whatever you buy on sale you use the regular price. Edie