doubling the cost of the materials does not pay you a reasonable amount per hour. your skill is worth more than minimum wage. figure out how many hours it takes and what your bottom dollar is per hour. add the cost of the materials. that's your baseline. you can make more money on small items because of the speed of the project.
what to charge depends a lot on where you sell. a double bed sized quilt of your quality in new york city in a quaint little store in an upscale area, for example, would cost in excess of $800. in a more modest area (say, a small town like mine) but still a suburb of nyc, it would sell for more than $500. in a working neighborhood, where people budget money to live comfortably, i don't know if it would sell at all. the garbage from china would be good enough. they really don't understand the difference.
if you seriously want to sell, find out how to do it in a tourist area or an upscale area. if you have to give a store a cut, build that into the price. think about running an ad in a newspaper geared to the "right" people.
have them call you for details re: size, colors, amount of quilting, etc. then you can give an estimate. think like a businesswoman. when you buy something you pay full value, right? you should get full value also. fair is fair.
please don't work for nothing. you'll just aggravate yourself.