Well, all prices are in Mexican pesos, but you can also pay with US dollars or by credit card. Joe's Oyster Bar gives a reasonable exchange rate for those who wish to pay in US dollars. However, the exchange rate will be less than you can get at a bank. So, we HIGHLY recommend that you pay with pesos! You can also exchange US$ for pesos at the cashier's window.
If it is not busy, you can usually run a tab with your waiter. When it is super busy, you need to pay when you get each round of drinks! We highly recommend that you do NOT run a tab! We have seen so many people run a tab and forget how many drinks they had. Then, when they go to pay, they get in an argument with the waiter over how much they actually drank!