6.38 Billion dollar USD.
But the most common form, and the one that PokerStars uses, is a % of the pot which is capped at a certain amount, based on the stakes being played. For example on PokerStars if you're playingR$0.01/$0.02 NLHE or PLO then the rake is 3.50% of the pot, capped atR$0.30.