Little late to the party on the response, but two major challenges with the idea of asteroid mining for use/sale on Earth:
1) Orbits. The fuel budget get a significant mass from the asteroid belt to Earth's orbit is ruinous. We are orbiting the sun at ~30 km/s. At the altitude of the asteroid belt, the orbital speed is ~19 km/s. So you have to increase the velocity of all the minerals you want to return by 50%. But first you have to slow it down, because you need to drop to Earth's orbit. But then you need to speed it up again to match Earth's velocity or else we'll reenact the extinction of the dinosaurs. And then you'll have to fight Earth's gravity all the way down to land it in a way that it doesn't vaporize in the atmosphere.
You could minimize fuel use by attempting very small changes in velocity, but then you're waiting decades or centuries for delivery and you still have to figure out how to land the stuff.
2) Economics. If you bring a billion tons of gold to earth, you don't get rich, you've just rendered gold worthless. There's no reward at the end.
As various people have said here, going up and down the gravity well is just horribly inefficient. Minerals in space will remain in space for use in space, and we'll just build mining/fabrication ships that utilize what we need where the mass already is.
www.teslarati.com
