That's a tough question because you can spend a bunch or basically nothing at all. It depends on what your intentions are.
If you want pictures by the onboard photographers you'll spend around $10 per picture.
Drinks run around $4.50 to $5.50 for mixed drinks and $6.50 for specialty drinks. Beer is $3.75 each.
Shore excursions run $25 to $100 and higher for some.
Souveniers can vary all over the place.
Slot machines are 5 cents to a dollar. Minimum on table games are $5.00.
Some ships have specialty restaurants that charge around $25 per person.
Tips are $10 per day per person for cabin steward, waiter, ass't. waiter. Fifteen percent on all bar purchases. A dollar or two per bag for the porters.
Massages run aroung $90 for a standard massage. Extra for the oils and such they sell.
Tips are expected for room service.
Tux rentals around $75.
In ports you can buy nick nacks. And drinks and/or lunch or snacks.
If you want to really splurge and do all the things available and drink whatever you want it can get expensive. But I know an elderly couple from Orlando who cruise about once a month on the Fantasy out of Port Canaveral who get last minute fares and spend nothing at all who cruise for around $250 each for a 4-day trip to the Bahamas.