We're talking about grams of raw dirt, not commercially interesting amounts of rare material that wouldn't be easier just to find on earth.
It would be nice enough to get them cheap enough that we could conceivably send enough to enough asteroids to determine if mining them was actually ever going to be worth it.
How much energy would it take to just, capture an asteroid and have it sat in the orbit of earth or some other consistently distant place, rather than having a mining operation on the asteroid as it zips around the solar system?
Could the latter be more economically viable than the former?
Capture might be done with gravity assists (via the moon) and aerobraking at earth, but it is still a significant amount of energy. It is probably easier to just mine deeper into the earth for raw materials.
There is an entire school of research associated with asteroid capture-and-return versus mining-in-place. I don't think it's a settled area of research, probably because we really don't know how to do at what cost either alternative.
It would be nice enough to get them cheap enough that we could conceivably send enough to enough asteroids to determine if mining them was actually ever going to be worth it.