My itemized onboard charges average around $350 for a 7 day cruise. That's just what I charge on the account. Those "Bucket of Beers" are not cheap. That does include my auto grats though.
Now the least amount for a 7 day cruise, counting the onboard charges, and what I play with in the casino would be $0. I won in the casino and paid everything off.
I have to save and be conservative all year for my cruise, so I don't worry about it too much once I step onboard. I budget $100 a day and almost always have money left.