What is the most expensive substance?

Antimatter. According to one estimate (Discover magazine, August, p.68), it costs $1,750 trillion per ounce to produce; here is another cost estimate. Gold looks absolutely cheap by comparison, and who cares if a possibly fake Vermeer just sold for $30 million?

Here are some people who think antimatter will become cheaper. So perhaps you should sell your stockpile now…!