You have to ask yourself, what is more valuable than the >$10,000 per lb cost of getting into orbit? Probably 10 times this for a Mars trip, and only doable in 50 billion-dollar lots to cover the cost of the trip, the mining operation, and the return.

Gold is, what, $600 per troy ounce. 20 of these to the lb, so $12,000 per lb. Barely worth putting in orbit. Certainly not worth bringing back from Mars!

Crude Oil is $35 per barrel. 6 barrels to the ton. so $200 per ton, $0.10c per lb - no way!

Diamonds? Platinum group metals - perhaps, at face value anyway. Trouble is, there's already a glut of diamonds, and De Beers runs a careful mechanism to avoid releasing too many onto the market and flooding the price. A billion dollars worth of diamonds or rhodium or iridum or whatever would lead to a glut in supply and a crash in price. Every made-in-China audio lead would be rhodium plated, and there would still be a glut.

Whenever a new mine opens up, to efficiently exploit some metal or other, older inefficient mines close down - squeezed out of business by the scale and efficiency of the new operation.

You have to turn it on its head. - Look back on the pioneer days. What was the value of Oregon Pine in New York? - Not much, and certainly not enough to bring it back across the prairies. But the value of Oregon Pine in Oregon was appreciable - for building log cabins!

Probably the most precious substance in the solar system is water (ice). You can make breathing air, reaction mass, and rocket fuel from it, and you can even build with it in the outer solar system.

The value of water on Earth is negligible, even in drought-ridden deserts, but the value of water in high orbit is incredible - at least the $10,000 per lb it would cost to get it up there to use it as fuel.

The value of water on the Moon is huge - 3-5 times this value. On Mars, it's also high, but we know that Mars has in-situ ice resources that can readily be utilised, if one had the infrastructure. It might be that we (or more likely our descendants) end up trucking water from Mars to the Moon by the megatonne, rather than sweating it out of moonrocks by the cc.

There's a huge literature on this field, discussing what might be valuable, where, in a future when solar system infrastructure is well-developed. But imagine describing to Orville wright or Hugo Bleriot that you could fly people across the Atlantic for less than a week's salary, if you only built a few mile-long runways and developed 100-ton heavier-than-air flying machines...