Your figures are impressive, but you haven't really calculated the total cost per kilo of finding and then moving finished product from the asteroid belt back to market. Remember, that cost does not just involve the delta-v to haul the stuff back to its destination, but the energy cost of boosting your extraction and production facilities out there to get it, plus the costs of operating out there.

Assuming this technology were already developed (which is where I suspect the real costs will probably arise), just the amount of energy needed to move a ship full of miners, supplies and machinery out to the Belt and keeping it supplied is probably substantial.

And it should be kept in mind, as in all resource activities, the exploration (prospecting) costs will probably outstrip the extraction (mining) and transport costs many times over. You have to find it first before you mine it.
As Walter Huston reminds the boys in "Treasure of the Sierra Madre", gold is valuable not just because you have to pay men to go get it; someone has to grubstake all the men that went out and came back empty-handed.

No, I haven't calculated it either, but I suspect the price of belt riches will never make it worth our while to go there and bring it here, even if it is neatly stacked there in ingots (or even finished industrial products) ready to haul back.

Of course, one can hope that advances in propulsion technology, and developing markets in space itself for this material may change the economics considerably, but it will always be a chicken and egg problem. How do you build space stations in the asteroids without asteroid mining, and how do you provide investment for your asteroid mines without space stations out there who will be markets for the product?

And it certainly can't be done if everything we need to get started has to be lifted out of earth's gravity well first. You need a functioning space infrastructure already in place to build on.

The major obstacle is the cost per joule to accelerate your vehicles and their cargo to and from their shipyards, their mines and then their markets. Advanced technology may drastically reduce the dollar cost/deltav-kilo, but the energy cost in Joules is fixed by physics.
There simply is no way around it. Everything you need will have to be accelerated there, and every gram of product will have to be accelerated back--even if the costs of finding and extracting it are zero.

Of course, we have no idea what technological advantages we may have available to us in the future.
But it might be interesting, as an exercise, to calculate costs and break-even points for a similar problem where aircraft (not steamships) are used to support a mining activity in Antarctica. We make the assumption (and its a stretch!) that someday spacecraft and space colonies and factories will be as cheap to operate as aircraft and Antarctic bases factories and mines are to operate today. At what point will the cost per kilo of the stuff we bring back break even?

We did it (with steamships and whales) a century ago, so we know the problem is solvable in principle. The question is, can we make money at it now?