If you're sending stuff back you need reentry vehicles. Even allowing for reusability, it's hard to imagine the cost of building and launching them being less than the value of iron they contain (current value around $1 per kg for the higher grades and a fraction of that for scrap) and you're going to have to deal with regulators to land them. Even if the asteroid material solves the problem of all the propellant needed as reaction mass for transit to and from the asteroid belt, the unit economics of return only make sense to bring back the highly valued elements, and only if the rare elements aren't actually ones where the terrestrial supply monopolist can ramp up production rates once they face competition.