In order to figure out how much money you will need, you should have an idea of where you will want to eat when you're at Disney. Are you just picking up a bagel for breakfast or going to a character breakfast. You could get bagels and beverages or cereal, fruit or even pancakes at a quick service location for $8-15 (both of you). If you're going to a character breakfast it will be over $30 plus a tip. For lunch, decide again if it's sit down or a la carte. You could get lunch at Casey's, Pecos Bill, etc. for $12-17. Same with dinner. If you go to a nice restaurant, plan to spend at least $20 A PERSON on just the entree and that's a modest price. We have easily spent over $100 in the evening on dinner for two. But, you don't have to spend that much!
I usually figure at least $20 per day for little things in the parks like water or ice cream.
Don't forget any tips for transportation to and from the airport and for the bellman, valet (if needed), housekeeper, etc.
Once you get on the ship, you don't need to spend very much. We stuck to mostly the free beverages and only bought two photos. We played bingo twice and all the rest of the money went to souvenirs. You have to decide what's important to you. If you like to drink (alcohol or even soda) then you're probably going to need a lot of cash on the ship. If you can get by with water, tea, lemonade and soda at meals, then you won't need a lot of cash on the ship.
I'm really good at budgeting Disney trips, so if you have any more questions or you want more info, just let me know!