I think both you and the store owner are going about this the wrong way.
Try this:- She supplies all fabric, batting, thread for a specific number of quilts. You make the quilts. She pays you an hourly rate. She displays the quilts for sale, at the same time advertising her fabric range with them, which will move some of those same fabrics she supplied. She charges what she wants, not your problem.
This is what I used to do and its a win win all round.