Context is everything. If you’re in a lab, Americium. If you’re on a lifeboat, water with toilet paper a close second.
Perhaps the definition should be:
Must be something that is actually traded or sold. I.e. there is a real market, and a definable price.
Price must be the lowest price it is legally available for anywhere in the world.
Is a substance, not a complex system or mechanism.
So street value of drugs is out. Notional production cost is out. Cells or organisms are out.
It looks as if tritium or some biological reagents are the front runners.
Quantum dots apparently go for between $3k and $10k per gram. But prices are very likely to drop dramatically as the technology improves and demand ramps up.
Another one that doesn’t quite meet my self imposed criteria. Vaccines. The active constituent in modern vaccines is often a killed or partial bit of a virus. So, it isn’t alive, and isn’t a complex entity. Really just a large bit of complex chemistry. So far so good. A quick look about shows up Hepatitis-B vaccine at a government buy price of $40 for ten shots, and each contains 10ug of active ingredient. So $400,000 per gram. Clearly this is an overestimate, at least insofar as it doesn’t price the rest of the vaccine kit. However there are other vaccines that are about $1.30 each shot, so if we price the active constituent at zero, for them, we might think that the active bit of the Hepatitis vaccine was worth about $300,000 per gram. Of course you can’t buy the raw active vaccine component. You have to buy the complete vaccine from the manufacturer, and they make the active component themselves, at a cost that isn’t disclosed. They might have a notional price for internal accounting, but since it isn’t traded, it fails my criteria. I also don’t imagine that the active agent is ever handled as a pure concentrate. But valuable it sure is.
Back to the moon rocks. Did a quick internet search. Cost of Apollo in todays dollars about 130billion total. Lets throw in Mercury and Gemini too since they were just research to get Apollo right. Call it a 160 billion US dollars. About 800 pounds of moon rocks collected.
So, moon rocks cost the USA 200 million per pound. If mining moon rocks had been top priority rather than sending people/safety, probably could have brought back 4 times as much (take one less astronaut there and back) so that brings it down to 50 million a pound.
Or lets hire the Russians to go get us some. 200 million to launch a sample return probe seems remotely plausible. How much could it bring back? One pound brings us back to 200 million a pound. 10 pounds, seems reasonable, brings it down to 20 million a pound. A hundred pounds, pushing it I think, and its down to 2 million. Another factor of 2 and its down to “only” a million a pound.
Looks to me like getting a moon rock for less than million a pound anytime soon is unlikely and its probably realistically a good bit more than that.
Even IF you could make space travel /sample return probe 10 times cheaper still, you are still talking a 100,000 dollars a pound.