What is a kilogram of asteroid worth?
If the product is sold on Earth (the only existing market) then there are two proposed types of asteroid ore - platinum group metals (PGMs) and iron-nickel metal.
They richest types of asteroid for PGMs have about ten times the concentration of these metals compared to ores being exploited here on Earth. This makes the PGM content of the asteroid worth about $3/kg.
Nickel is worth much more than iron, and iron-nickel asteroids have a nickel content ranging up to 25%. Exploiting a nickel rich body, with 25% nickel, at current market prices (for pure refined nickel) would be worth no more than $3.50/kg (but a good bit less in truth, as it is really a crude ‘master alloy’ suitable only for making alloy steel rather than the higher value uses of pure nickel in batteries).
Currently there are only three mines on Earth with ores worth more than $1/kg — the Cigar Lake and McArthur River uranium mines operated by Cameco, and the Fire Creek gold mine operated by Klondex with ore values of $8.50, $5.60 and $1.60 per kilogram respectively. So the fabulous asteroid ores are actually only roughly as valuable as the richest ores on Earth.
Normally mining concerns profitably produce PGMs, and iron and nickel by having very low cost bulk processing methods that cost only dollars per ton of ore on ores worth pennies per kilogram.
The only asteroid retrieval mission so far proposed and costed is the Asteroid Redirect Mission:
Asteroid Redirect Mission - Wikipedia
which proposed bringing a 500,000 kg asteroid into Earth orbit at a cost of $2.6 billion, or $5,200/kg. Even this price does not get the asteroid to the Earth’s surface, in a marketable form.
Reductions in launch costs will not put more than a very small dent in this estimate as only a single Atlas V launch is required, which costs a mere $110 million.
There is no plausible way that asteroids can be safely diverted to Earth’s surface for a cost less than the value of the ore content. Not even zero launch costs from Earth look likely to change that due to the costs of extracting and returning the material, so appeals to “cheaper launches in the future” are a red herring.