There are several extremely expensive substances in the world, and all of them have to do with exotic physics. Historical contenders for the most expensive substance such as saffron, plutonium, gold, and diamond are like dirt in comparison to these. Three categories of material that compete for the most expensive substance are antimatter (especially antiatoms like antihydrogen), at $1,750 trillion US Dollars (USD) per ounce, followed by hafnium and tantalum isomers, at about $480 million USD per ounce, and assorted exotic particles such as bottom quarks, which are in the same league as antimatter.

The cost of antimatter has actually come down. In 1999, it was about $62.5 trillion USD a gram ($1.75 quadrillion USD an ounce). The reason antimatter costs so much is that it can only be made in tiny quantities using multibillion dollar particle accelerators. When tremendous amounts of electromagnetic energy is concentrated in one place, both conventional matter and antimatter are spontaneously created, and physicists can use magnetic traps to isolate antimatter. Though they usually create just antiprotons, they can even combine antiprotons with positrons (anti-electrons) to create anti-hydrogen.

Antimatter is worthy of the title of most expensive substance because it is also potentially one of the most useful substances in the universe. Antimatter has the theoretically greatest energy density out of any energy source. The energy in 1 kg of antimatter is equivalent to 47 megatons of TNT, similar to the largest nuclear bomb ever detonated, which had a mushroom cloud seven times taller than Mt. Everest and could have caused third-degree burns at a distance of 100 km (62 mi). Antimatter has such a huge energy density that, if it could be harnessed for rockets, it could accelerate a payload to a substantial fraction of the speed of light. Few other propulsion technologies are capable of this, even in principle.

The most expensive substance besides antimatter is other assorted exotic matter, as mentioned before, as well as hafnium and tantalum nuclear isomers. A nuclear isomer has the same number of protons and neutrons as the normal element, but one or more of its nucleons is in an excited state whereby it contains greatly more energy than the element at its ground state. Such isomers can be induced to emit gamma rays, and at one point were eyed as a futuristic fuel or bomb material, yet induced emission of nuclear isomers has yet to be developed in a reliable way. Unlike antimatter, this most expensive substance is more useless, at least until technology progresses somewhat.

I don't think you could sell it on ebay. nobody is that rich :p

Dark matter is more expensive.

Dark Matter is 22% of the universe and can't be used for much else.

If you can't get any antimatter, try to get some bottom quarks. (Half the mass of the inside of a proton)
They're worth a fortune too.