The way I like to think of it is that Google has a wondrous faucet that constantly exudes liquid gold, about a cubic centimeter every second. It is our job to keep the faucet open. Occasionally we also do other cool stuff.

(19.32 gr/cm^3)(37.88 $/gr)(60 * 60 * 24 * 365 s/year) ~= $23B, close enough.

The density of gold at its melting point is actually 'only' 17.31 g/cm3.