With a mass of 40 milligrams and most recent auction sale price of US$ 8,307,000, the unique British Guiana 1c magenta stamp sold for US$ 207.7 million dollars per gram. It’s important to distinguish objects from substances. Arguably, the most expensive substance would be antimatter (if it were possible to create and store in bulk)—NASA estimates it would cost around US$ 62.5 trillion per gram, which is one of the few numbers you’ll find after a dollar sign which is larger than the U.S. national debt. Other extremely rare elements and isotopes such as astatine, of which much less than one gram is believed to exist in the Earth’s crust, might qualify, but even if you collected a bit of it, it’s unlikely you could sell it for much since it has no practical use and decays with a half-life of 8.1 hours. Here is a list of the rarest substances: only antimatter comes in above the legendary stamp.
Of course, in this age of NFTs and securitisation of everything, the present owners of the stamp are peddling fractional ownership shares in the stamp to the public.