The radioactive poison, Polonium 210, used to murder spy Alexander Litvinenko, is a lethal but hugely expensive substance to manufacture
It’s a byproduct of certain reactors. So manufacturing it is in fact free.
The polonium used to kill Mr Litvinenko would have cost “tens of millions of dollars” if bought on the open commercial market.
Because almost no one bothers to process it out of the wastes. it’s one of those things that, if you actually wanted any quantity of it, you’d have a chat with the people who do nuclear waste reprocessing and you’d get as much of it as you wanted for peanuts. Once you’d set up the extraction system that is. The current cost is imply because almost no bugger wants any.