You really don't want to go all the way to Mars for nickel or iron :-). Now if chunks of platinum, gold, silver, and palladium are laying about to be picked up, well that is a different story. 100kg of that stuff my be worth 3 - 5 million $ when you got it back here.

On the plus side, if you don't need to smelt iron ore to get metallic iron you can build stuff on Mars without first building a massive processing plant. (steel mills are smaller than iron smelters in my experience)