The monetary density of things...or what substances are worth their weight in gold, including $50 bills, LSD, and antimatter.
People have been saying that the new industrial grade swimsuits like the LZR Racer are worth their weight in gold. As you can see, this is clearly inaccurate. But such a suit is worth its weight in marijuana or industrial diamonds.
Using a FAQ at NASA, I calculated that a pound of aerogel is worth about $23,000...more if the aerogel has a particularly low density. One other note: printer ink is more than 50% more expensive by weight than silver is. (via mr)