I think nickel-iron is an extraordinary and perhaps unique resource that is (by Earth market standards) valuable even as a raw and unprocessed commodity - perhaps several thousand US$ per ton for high nickel content would seem likely, with potential value in the precious metals mixed in probably exceeding that. But being mixed in at low concentrations in metal alloy, extracting and refining looks to be very challenging.
Rather than attempt it in space I think delivery to Earth might offer an alternative that minimises in-space costs. Relatively simple processing would be needed to extract nickel-iron - possibly from metal rich chondrites that have metal as grains or nodules within softer carbonaceous material. Or perhaps shattering at very low temperatures could be a low energy, low cost method of mining bulk nickel-iron.
Because of the costs of launching bulk cargo to space is so high I think any attempts to mine asteroids must ruthlessly limit the equipment and materials launched from Earth and rather than attempting to establish refineries to extract the high value elements from nickel-iron it may be better to deliver nickel-iron to Earth.
But can bulk "hard" materials be transported effectively (perhaps fueled by asteroid water broken down to H2 and O2) delivered at low cost to Earth's surface? How?