I went on a 7 night cruise and I think I spent like $50 on the ship... you'll spend more if you want to drink, and of course they give you ample opportunities to spend your money, but it's not like they hold a gun to your head. They'll gouge you on the excursions, but we mostly went out on our own. They were charging $150 for one excursion, and we basically followed the excursion's plan, took public transportation, and did it for about $30.
I know some people could just stay on the ship the whole time, but I personally got far more out of the ports, and probably wouldn't spend that kind of money to just relax on the ship. Granted, there was a lot to do, and the endless supply of food was nice, I got kind of restless during the days at sea. Probably just me though... doing nothing on a balcony of a ship is great for 15 minutes or reading for an hour is nice, then I wish I had an internet connection or ps3.