I think the real value is amount of rare metals that could be harvested, scientists have found an asteroid that is comprised mostly of metals. Scientists think it may be the exposed core of a proto planet:

Metal asteroid Psyche has a ridiculously high 'value.' But what does that even mean?

So that kind of haul could potentially be worth it, but smelting, refinement and processing would probably be more cost effective in space. Who knows what the future will bring, mining the asteroid belt may only make sense once we have a much strong presence in space, I.E., colonies on Mars, the Moon, etc etc.