When looking at the rules, mithral costs 500 gp/lb. Heavy armor needs another 9000 gp to the base price to be made of mithral. A Fullplate weights 50 lbs. The rules state that one has to multiply the original weight with the costs to get the price. Now combining it with the price per lb this would be 25000 gp. The rules also state that an item not primarily made of metal doesn't gain any benefit from being made of mithral. So no other (cheaper) materials for the mithral fullplate could be used.
So something is odd… what is the real price of mithral per pound? Or why do the values for item creation differ so much from the "real" value?