Commercial asteroid mining will be reality within this century, and is already attracting investment. Look up Planetary Resources & friends.

There are individual asteroids that have more easily extractable precious metals than the entire present earth supply, should we have a good method of returning them to earth. The mere existence of these ought to start playing a number on the metals markets once planetary resources starts doing something newsworthy.

People who I assume know much better what they are talking about (space agencies, scientists) have already commented on these wild space mining fantasy you are referring about. It's a ridiculous idea that is nowhere near technical or economical viability, and there really is no horizon on which it will be. To mine asteroids you would first have to build a permanent moon base, and drive down the costs of space missions down to the point that a single payload brought to earth would be worth more than the required investments.

Just because some nutters invest their excess money in some sci-fi fantasy doesn't make it real.