Nickel is so common and cheap it is unlikely to be profitable to ship it to Earth. It's more likely that nickel and iron would be used as construction materials in space, to make satellites and stations cheaper by reducing the launch mass required.
The profitable stuff to ship to Earth would be platinum-group metals, which have values in the tens of thousands of dollars per kilogram. But those require much more capital investment, as you need to get to farther-off asteroids and do more intensive in-space refining.
