It is math- for the booze. We few people would realistically spend $60 (or $120 a day for a couple) on non-alcoholic drinks. But you do not have to get the 15 drinks to justify the purchase. Even at three drinks, some might find Cheers worth the price. Consider the average price of a drink is $12, then add two coffees at $4, a couple of bottles of water and cans of soda at $3 each, and a shake or frozen drink at $7 then you would be at the $60 break-even point.
Additionally, there is a lot of sharing on Cheers. I observed this (and benefited) firsthand on my recent cruise.