The steel molds are the "tools" in question here. They cost a fortune to make.
It still costs money to make them for things like armor, heads, etc. The difference is that it either takes fewer of them, or smaller ones (depending upon whether or not they put multiple items/parts in a single mold).
A fully tooled figure costs more because in addition to the things like the armor, head, etc. (the things that are unique to most average figures), ALL of the other parts are brand new as well. So there is more cost involved. It's not like there is just one set cost for tooling no matter how many unique parts have to be tooled. It varies depending on the size, quantity, etc. The fewer unique parts there are to tool, the cheaper the cost.