How much time does it take YOU to make one? Be sure to include the time it takes for preparing the yardage (pressing/starching), cutting, sewing, then pressing the finished block. I would charge a minimum of $10/hour on top of the price of the materials. The quirk is that you probably wouldn't make one at a time, so you would chain sew, make multiple HSTs etc. so it will probably take less time per star, the more stars you make.
Then you would also have extra time invested if you added sashing and border(s) quilting, binding...