Lots of people all put the same amount of money in each and then someone goes to the bar to get the drinks with the combined money. What is that called?

I am trying to figure out how much money I will need to save for a 7 day cruise on Carnival. My fiancee and I are average drinkers (probably 4-5 drinks per day) and would most likely go on about half the excursions that are offered. Given that information, can anyone give me an estimate of about how much money I should be saving?

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