Well the VALVES OF MITHRAL are easy, they have a cost per pound listed. They're 14' tall x 28' wide x 3' thick so 1,176 cubic feet. 1 cubic foot of steel weighs 490 lbs. Mithral is half the weight of steel, 245 lbs per cu. ft..
1176 x 245 = 288,120 lbs of mithral
288,120 x 500 gp/lb. =...