If you're referring to the earth's iron-nickel core, then the comparison is moot. Mining an asteroid is theoretically feasible. Mining the earth's core is not.
Hum, no. The crust is full of Fe already. It is probably much cheaper to get it from dirt than from this asteroid, but it's concentrated on a huge amount of places.
If it was about the core, the answer for Ni would also be obvious.
> It is probably much cheaper to get it from dirt than from this asteroid
Well, delivery costs play a role as well.
If you're looking for iron for use on Earth, you're right. If you're looking for iron to use in space construction, the asteroid material is likely much cheaper.
If one believes (as I do) that the human race will either move into space or go extinct, then asteroids will be far more important for the future of our society.
If it was about the core, the answer for Ni would also be obvious.