Many newer ships have alternate restaurants which have a surcharge. Monarch does , so both dining room and buffet offer food that is included in your fare.
What you might spend extra is the toughest question. Alcoholic drinks are extra, as are sodas. So, it all depends on what you're going to drink. Same goes for sovenirs.
As for Jamaica, you can certainly bargain, and bargain hard for any purchases. The vendors expect it, and enjoy the game as much as you might.
As a general rule I would count on an extra $100 per person, per day in expenses over and above your cruise fare, and cost of any shore excursions you may want to do.