It's far cheaper to mine here on Earth. You could explore, prove up, and develop a world class ore body in any commodity you care to mention for the cost of one shuttle launch.

Other issues: we are not running out of any metallic mineral commodity, despite unprecendented consumption. We don't know how to mine in zero gravity, this includes the entire processes from excavation through milling, smelting, and disposal of waste. We don't have the technology to economically explore asteroids, let alone economically move thousands of tonnes of material round in space, both setting up the plant on the asteroid and then shipping it back to Earth.

We have got to crawl before we can take bady steps. As far as space mining is concerned, crawling will include being able to excavate useful amounts of lunar and martian regolith for construction purposes. Then we can move on to volatile extraction and then the harder things - lunar oxygen, production of iron and tiantium and aluminium. By then we may be in a position to start thinking about mining the asteroids. Even there, the most vaulable resource is not metals, but volatiles. Volatile extraction on Phobos and Deimos will be hpow we learn to work on asteroids.

None of this is for the terrestrial economy, all is to increase the efficiency and reduce the cost of space operations. Mining asteroids for metals and exporting them to earth is economic fantasy for a long time to come.

Jon