Look at the delta-v required to reach various asteroids. The near-Earth ones are lower than most.
Look at the cost of delta-v per kg of material.
If your target were Mars, and Mars were paved with gold, then gold at $1,800/troyoz is only worth $57,870/kg, meaning that if Mars were paved with gold, it would cost you $942,128 per kg after selling the gold to bring it to Earth.
Then factor everything else into your equations. You'll need drilling and prospecting equipment, you'll have exploratory trips, if you plan on sending up humans you'll need life-support systems and their added mass.
Quite simply, there's nothing material in space that's going to be less-expensively procured for Earth on Earth.
Communications, surveillance, exploration, and research would be the exceptions.
Cost per delta-v is dropping, hopefully by an lot if/when we get reusable rockets. Given that near earth asteroids require dramatically less delta-v than Mars (because no gravity well) and some are high in platinum, iridium and other expensive things it sounds like mining asteroids could be cost effective soon.
I'm not arguing with your numbers, but could you spell out your calculations a bit more? How are you calculating the cost of delta-v per kg?
There's presently no way of constructing or provisioning a rocket entirely in space without first lifting materials (or the entire craft) from Earth. Should there be, whatever costs are associated with that will shift the equation here.
In the meantime, you're stuck with the reality that whatever mass you plan on transporting to the asteroid, and whatever mass is required to haul back your loot, needs to be boosted from Earth's surface to LEO.
The tyranny of the rocket equation dictates what delta-v costs you. Trips with burns at both ends are vastly more expensive than those with burns at only one. So, yes, aerobraking (on return to Earth) is a highly cost-effective method, but that's going to require budgeting for the mass of your reentry shielding and landing mechanism (likely parachutes).
Ultimately, you're looking at a high-speed, high-temperature reentry, atmospheric slowing, and an ultimate soft-ish landing of whatever you've recovered.
While ion rockets have been proposed as vastly more mass-efficient than chemical rockets, existing designs based on xeon rely on an element whose prevalence in known space environments is quite low. The one exception is Jupiter, but that's the second deepest gravity well in the Solar System:
Within the Solar System, the nucleon fraction of xenon is 1.56 × 10−8, for an abundance of approximately one part in 630 thousand of the total mass.[53] Xenon is relatively rare in the Sun's atmosphere, on Earth, and in asteroids and comets. The planet Jupiter has an unusually high abundance of xenon in its atmosphere; about 2.6 times as much as the Sun.
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.
Oh, and per the reference I pulled up, cost per pound wasn't falling appreciably by 2000. Yes, we're another 14, call it 15 years further on. But as the chart on page 4 of the Futron report shows, aside from a big drop in 1992, there was effectively no movement during the 1990s.
Cost is dropping, but even if/when we reduce the costs by 10-20 times (as expected by the rocket reusability plans) then it's still unprofitable.
Even if Santa gave us a free colony of Mars and the rocket costs dropped as much as we hope and Mars was covered with bricks of platinum and iridium - even then it would be economical to simply leave it alone.
Look at the cost of delta-v per kg of material.
If your target were Mars, and Mars were paved with gold, then gold at $1,800/troyoz is only worth $57,870/kg, meaning that if Mars were paved with gold, it would cost you $942,128 per kg after selling the gold to bring it to Earth.
Then factor everything else into your equations. You'll need drilling and prospecting equipment, you'll have exploratory trips, if you plan on sending up humans you'll need life-support systems and their added mass.
Quite simply, there's nothing material in space that's going to be less-expensively procured for Earth on Earth.
Communications, surveillance, exploration, and research would be the exceptions.