bringing hundreds of tons of iron or such between each is one of the most un-ridiculous things I can think of, and it would be done.
Why would it be done?
We've got plenty of Iron on this planet already.
More generally, there's very little on Mars that we don't already have in plentiful supply on Earth.
If you go down to the basic mechanics:
- Energy costs money.
- Elevating mass out of a gravity well takes a lot of energy.
The natural result is that elevating all but the rarest of resources will always be cost prohibitive.
There's one exception to this; fuel.
It's always cost effective (though ludicrously inefficient) to elevate fuel from a gravity well.
I believe using Mars as a fuel refinery has long been considered the prerequisite to any viable Mars colonization effort, as it drastically cuts the outward journey costs if you don't have to carry your return fuel with you.
I think Mars will be one of the latter planets/moons to be colonized by Humans, as it has few benefits, and many drawbacks, to living on Earth.