Asteroid mining
Podrock is right that it isn't cost effective now to mine asteroids, but at one time it wasn't cost effective to pull oil up from a mile underwater either.
I wrote a paper years ago on the subject that you can pull up (file no. 18 from the old board's gallery, not restored) in PDF form.
The data is old, but probably still in the ballpark. The stuff is out there, and in huge quantities. Someday we'll go get it.