Asteroid mining is economically unfeasible and likely to remain so for at least another 50 years (possibly forever, as reclaiming rare metals from road-dust and recycling electronics is far more practical). There are three main costs to any kind of mining:
(1) exploration/resource discovery, and ore vein mapping: This will require identifying a target and landing something on the
asteroid/comet a project very similar to that undertaken by the Rosetta
(Philae) project at the cost of $1.8 billion USD.
(2) machinery & fuel for extraction : This has completely unknown costs since current technologies all rely on the
presence of oxygen to burn fossil fuels powering drills, and setting off
explosives. That's just for getting the ore into transportable chunks.
However often ore will be processed to increase the concentration of the
mineral of interest before being transported long distances to cut down
on transportation costs (known as smelting). Current smelting
techniques typically require high temperatures and/or liquid water,
neither of which are readily available on asteroids.
(3) transportation of extracted material : This would require something like a shuttle to protect the minerals from
burning up in the atmosphere on the way down. A close analogy is NASA's Shuttle
program which cost ~$450 million per trip, which given the current
price of gold would require 12,000kg of 24 carat gold per trip just to
break even. The payload capacity of the shuttle was only 22,700 kg which doesn't leave much margin for impurities or extraction/exploration costs.
This means it is a vanity project for all the many super-rich investing in the various companies 'planning' to engage in asteroid mining. It is a disgusting display of wealth which highlights the vast inequalities of our modern society. Notable 'investors' in asteroid mining are Larry Page and Eric Smidt of Google, and some former execs from Goldman Sachs: people whose fortunes have been amassed by invading our privacy and selling that information to advertisers, and who probably contributed to crashing the global economy. While historical aristocrats invested their money building vast houses befitting of the title 'estates' or on extravagant monuments, the current generation prefer throwing it down science/technology pipe dreams. However, unlike the Great Pyramids of Egypt or Europe's many castles there will be nothing substantial left behind for people to gawk at or to be re-purposed into hotels or conference centres when these companies inevitably fail. Thus, these sci-tech vanity projects are more similar to the roaring parties of the 1920s, perhaps justifying the recently invented term 'nerdgasm', than the extravagant constructions of the past.
Unfortunately the amount of investment in these projects is not public knowledge so I can't calculate the number of lives that could be saved if the same amount were to be invested in malaria fighting bed-nets ($10 a piece) or vaccines to eliminate polio (<$1 billion/year globally) or providing maternal health care or paying for anti-retroviral drugs or building toilets or providing a source of fresh water or funding schools etc... But no, unlike the Gates', the 'cool' techno-aristocrats prefer to flush their money down a giant space toilet.
