I don't know for the shadow golem but the entry for Iron Golem gives us this :
We know that iron cost 1sp per pound. So, that's about 500 gp for the raw material. You can then reduce the cost of the Golem by 500 gp if you find a way to get free iron.An iron golem’s body is sculpted from 5,000 pounds of pure iron, smelted with rare tinctures and admixtures costing at least 10,000 gp. Assembling the body requires a DC 20 Craft (armorsmithing) check or a DC 20 Craft (weaponsmithing) check.
(...)
Cost 80,000 gp + 5,600 XP
If Shadowiron's price is higher than normal iron, then the economy might be greater.
It sounds logical to me, at least... but I'm not the more rational mind around.