If you've never been on a cruise before then there is a lot to know Before You Go. Cruises are a very different type of vacation.
First, when calculating the expenses of a cruise there are two big considerations:
The food is included. There is usually food available 24/7 on a cruise and it's (almost) all included in the price of the cruise. There are alternative dining options, but the free food is usually so good that it can be hard to justify paying more. Drinks are not included beyond water.
Tips are mandatory. Although the food is included, so are tips. The tips cover your room service, restaurant tips, your room steward, and other general tips. Keep in mind these tips tend to be standard even if you don't use the services.