Whether it's prostitutes or mistresses doesn't matter. One is paid per "trick" I expect unless she is "on staff" and other is paid for on a continuing basis I expect. Billionaires must have very big budgets for such things. You don't even have to be a billionaire. You can be a sleazeball millionaire like Harvey Weinstein. It costs them. They pay to play. They gotta. It's the only way they can get women.