A standard penthouse will set you back $1.5 million in game (a little over $20 in shark cards), with all the optional extras added, such as private blackjack table, bar, spa, arcade machines, etc bringing that price up to around $6 million instead.
This functions in the same way as other apartments/living spaces in the game, with each player gaining access to his own living area in which they can invite other players and customise how they see fit. What would a casino be without a place to live to keep you close to the action? The Diamond offers this in the shape of the Penthouse suite.