It's cheap to make from asteroidal material and will be in great excess. You don't need heavy duty high thrust rockets. Simple, small, low thrust rockets of extremely high reliability are quite sufficient, and they are reusable. The carbonyl process is very simple, and separates iron, nickel, cobalt, and the platinum group metals very nicely. Asteroidal material by its very nature is quite useful and valuable.
Asteroidal material is relatively easy to process in space.
The rest of this page goes into details on the above scenario.