I can imagine a sort of a conveyor belt made of miniature cargo vessel with one robotized mining station at one end, cutting away an asteroid piece by piece, and a cargo dock at the Earth side.

With enough cargo vessels deployed, let's say one would arrive at each end everyother day, the moment the conveyor belt was full, the mining operation would be swift.

Assuming a global deal between nations could be struck to have a refinery or at least a cargo dock placed on the moon, to organize large cargos to come to Earth at programmed intervals, it could prove to be a very interesting endeavour.

Raw matterials price could drop, given the sheer available volume.

At least it sounds like a diferent sci-fi plot