alt Hacker Newsit is like for example situation with energy production 30 years ago when solar was expensive and coal was cheap. Yet the solar have been becoming more and more cheap while coal becoming more and more expensive. The same will be with a lot of mining - the cost of it on earth, including environmental costs, will be rising while space mining costs will be falling. It will take time before space mined iron would make economical sense, where is space mining things like previously mentioned iridium, etc. looks to already have economical sense, or very close to it, even today.
Space iron? Earth's crust is full of it, it will never make economical sense to bring it from space. You could use the energy needed for a rocket start to melt and seperate it from allmost any rock here on earth way easier.
Some very rare elements or tritium maybe, but this is is a big maybe.