Width of the fabric is also a consideration. If the fabric is cotton, of nice quality AND the standard 40 to 44" wide then I would say he could ask for $30 to $35 per 3 miter piece, maybe even more as it is for a very good cause. BUT if the fabric is a narrow width, say only 30" wide then his $15 $20 per 3 miter piece would be in line assuming of course the base fabric is a nice cotton. By the pictures you posted it looks very nice and I too would love to purchase that red with the white swirls.