My usual method is to think of a price that I would charge for my time and labor (but don't try to figure this out as an hourly rate - the quilt would get astronomically expensive if you paid yourself a living wage!) and then I add the cost of materials to that. By doing it this way I give the customer a ballpark number (say, $300 for a queen) which goes up depending on their fabric/batting/backing choices. I have had people who really only care about the front of the quilt (eg for a wallhanging) so poly batting and muslin backing are perfect, and the final price reflects those (lower) costs. Others have wanted a fully reversible, all-cotton quilt, with a pieced back, or one made from an atypical backing fabric - and the price goes up accordingly. The initial price I quote would also take into account hand vs machine quilting, and any embroidery or applique they would like.
Good luck! It is a great feeling to be able to support you fabric habit...
Alison