Asteroids are hit-and-miss composition wise (though you can determine quite a bit by observing them), but when they are a hit they tend to be really valuable. The problem then is that you need to get to them and then the next problem is that you need to get back to where you came from.

Both of these can hurt your ROI considerably assuming you can solve for them at all with the masses involved. They're also usually moving at a pretty good clip and are bad to set up long term for. I think until we have a long term presence in the asteroid belt that this is mostly going to be SF rather than that it will actually happen.