What is your definition of binding? Will you provide the binding or will the customer provide the binding? If the customer provides the binding will she cut it and sew the pieces together and press in half, then wind ready for it to be put on the quilt. Will the customer machine stitch the binding to the front of the quilt and you will only hand stitch it to the back? Or, will you machine stitch to front and hand stitch to back?
Assuming all you will do is hand stitch to the back, for a 108" square quilt you would have 432" of binding, divided by 12" equals 36 feet times 18 cents a foot, you would make $6.48 to bind a king size quilt. I would certainly send my quilt to you for that price but don't think it would be worth your time to do it.
I hate the hand stitching part of binding and would gladly pay someone to do it. However, it would have to be reasonable but believe that reasonable would be more than 18 cents a foot.
Good luck in your decision.