Sure I get that. Landing via parachute remove almost all the return cost. Leaving the cost of a mechanism in space that can run for years, delivering asteroid metal to earth. Its disingenuous to claim a fixed startup cost prevents profiting from what essentially becomes an industrial infrastructure. It costs billions to create a new oil refinery, yet we do it all the time.
Your delta-v for return may be low (60 m/s), though most are higher (see below), but that relies on finding near-earth asteroids with favorable mineral characteristics. You can reduce the mass you're returning _if_ you can refine or reduce it on-site, but that requires additional mass to be transferred out.
Orbital mechanics are far from my forte, but none of this comes cheap, and you're still stuck with costs in the order of $5,000 - $10,000 / kg for Earth to LEO. Which includes the mass of your vehicle, its fuel, and any mining equipment your lugging around.
A catalog of 11,834 NEOs as of yesterday maintained by NASA / JPL shows a minimum delta-v of 3.8 km/s and a high of 26 km/s. Mean is 7.97 km/s, median is 7.1 km/s.
Every mission doesn't have to start from the ground. We don't build a new refinery for every tanker. Make your factory in orbit (or better yet- near the asteroid field). Send the refined metal back.
The deal is, deflect just ONE asteroid to earth orbit, and that's maybe more metal than our civilization has mined so far. The potential is, a new order of society here on earth. The cost - some billions.
"Every mission doesn't have to start from the ground."
I've already addressed that. We do.
"Near the asteroid belt" is meaningless. "The asteroid belt" is huge, and "near" in this case would mean "within a low delta-v orbit". Either way, you're shuttling raw ore or the refining equipment.
There's actually an avenue you haven't proposed: utilizing the asteroid directly for propulsion. There are a few possibilities, including laser ablation (there's a recent PhD thesis on this proposal: http://theses.gla.ac.uk/5219/1/2014GibbingsPhD.pdf ), "pebble drives" in which a mass launcher ejects loose material from the asteroid directly (creating potential collision hazards for other craft, though space is big, really mind-bogglingly big), or solar sails using sunlight to alter orbits gradually over a long period of time.
