But what is the actual value of something like this? Surely something comparable has happened in the past where a big reserve of some rare mineral has been found and monopolized, and its price didn't go to zero.
In fact, the actual value of a hard to get Fe reserve is 0. The ground is full of it, and it's much easier to get here. We would have to compare the costs of mining an asteroid with deep mining on Earth to discover if the Ni has any value, it's very likely 0 too, but I don't think anybody knows for sure.
If it was about the core, the answer for Ni would also be obvious.
Perhaps I misunderstood. What do you mean by "deep mining" for nickel?
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.
...and so would the prices of everything else. As well as the number of life-bearing planets in the solar system.
Imagine the cost of launching that much iron into space...
The rise in prices made Spanish exports uncompetitive, forcing Spain to spend its bullion to buy finished imports from other countries.
Aluminum. Initially so expensive that the tip of the Washington Monument was made with it, then price plummetted after new refining methods.