Well, let's be cautious. The constraint of moving from the asteroid belt to near-Earth orbit is less "absurd amounts of energy" and more, "how fast do you want

Well, let's be cautious. The constraint of moving from the asteroid belt to near-Earth orbit is less "absurd amounts of energy" and more, "how fast do you want to get there?". More importantly, we're not necessarily going to choose the asteroid belt for our source, due to these time constraints. Measuring in terms of Earth's orbital radius ("astronomic units" or AU), the Moon is 2.5 mAU away, Mars and the asteroids are 1000+ mAU away, and the metallic asteroid (6178) 1986 DA approaches Earth's orbit within 161 mAU, which is to say nothing of Earth-crossing asteroids. It's true that moving an asteroid would require a lot of energy simply because they're unbelievably massive; the space shuttle propulsion system might dish out something like 10^14 joules, but a typical mining asteroid is 10^13 kg or so. But apart from that, space is frictionless, and so moving a longer distance does not require much more energy, just much more time. Sending ships to and from asteroids shouldn't require an energy cost except to cut down the travel times, in principle.