I think you may have figured incorrectly. A 50" x 50" quilt would have a perimeter of 200" which equals 16.7 running feet. At $.15 per running foot you would only make $2.49!! (16.7 x .15).
I charge by the linear inch. If I'm making binding, machine attach to front, hand stitch to back, I charge $.25 per inch. So in the above example I would charge $50. Maybe I'm slow, but it will take me at least a couple of hours, and I won't work for less than minimum wage.
A business needs to consider that the amount the business takes in (the $50) is NOT the amount the 'employee' (you) makes. You need to subtract out overhead like advertising, supplies, self-employment taxes, etc. General rule of thumb is 1/3 to taxes, 1/3 to overhead, 1/3 to yourself.