Asteroid mining starts to make sense when someone like SpaceX succeeds with full-rapid-reuse rockets and gets launch costs to $50/kg. For a trip to an asteroid, probably a few hundred per kg.

At that point, mining asteroids for rocket fuel is less of an advantage, but mining them for precious metals makes more sense. Gold is $54K/kg, platinum is $30K/kg.

And sure, they'd get cheaper if someone started importing them from space in sufficient quantities. But we're already mining about 2500 to 3000 tons of gold every year, so an asteroid miner could make a lot of money before crashing the price.