The idea of mining asteroids IS kinda ridiculous considering we have far more material right underneath our feet.
Ya but they will only become worth mining when it becomes cheaper to get the minerals from asteroids then from earth.
Well, spaceflight is expensive now because the spaceship are build on earth and it ask a lot of energy for bring them around our planet...
Once we have some space dock around earth, resource ( metal from asteroid, hydrogen fuel from gas giant, etc ) needed for build spaceship will be cheaper from outer space... asteroid have a gravity almost to zero, if you wish, you can simply kick a container and soon of later, it will reach the shipyard around the earth... you have no friction from atmosphere and no huge gravity to fight continuously for space transport...
As today, send 1 ton of steel in space is very expensive due to the transport cost... send 1 ton of steel from a far away asteroid to the gravity wheel from earth will cost almost nothing...
The problem is that we need first to build a few "factory" around the planet and around place we wish to mine resource... once this is done, money will begin to flow... in long term, mining the outerspace can generate profit, a lot of profit but the initial investment will be huge... and since the actual economic trend is immediate gratification/benefit without long term view, it will not happen soon...