Adamantite is rarer than mitril. Yet, the offer was 100 large wagons each of which is packed full of adamantite for 40,000 gold coins total. That comes to a MERE 400 gold coins per large wagonload of adamantite metal. That is pretty cheap adamantite because it is rarer than gold. Thus, the 400 gold coins should buy LESS adamantite metal than the physical weight or volume of the gold content in those those 400 gold coins.