How much spending money does one REALLY need to go on a cruise?

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I'm going on a cruise with my wife, daughter and my mother-in-law at the end of January and I've never been on one before so I started reading about the cruise line and you know, what's available, etc. In all the reading I think I got MORE confused about what to do, what costs what and when to pay, who to pay and so on. Has anyone ever been on a cruise, specifically with Princess, that can sort of explain the process to me? How much spending money does one need to have a good time (nothing fancy, just overall good fun), not including the cruise fare... I'm talking strictly spending money for food (other than what's included, which is another confusion as apparently some stuff is included but others are not), casino, shows (unless it's included, it never said), etc. Anyone.... help?