The reality is that cruises can (and will) nick you at every turn. How much is up to you. Spending an extra $50 per person per day is fairly average, and it's easy to drop a whole lot more.
"The major extra is alcohol," says Brian David Bruns, who wrote about his year working as a cruise waiter in Cruise Confidential. "We were always puzzled by passengers who stayed on board drinking $6 Coronas when they could buy them for a buck apiece at a bar in port."
Another way to save: Ask a friendly crew member to tell you about excursions you can book on the spot in port. They generally cost half of what the cruise line charges.
