Well I have a cruise template in excel format. I drop the costs in there.
Things I include:
Tips (Carnival charges $10 per person per day),
Taxes and port fees,
Pre/post cruise hotel costs
Money for drinks including sodas (they charge for soda on board),
Money for shore excurisons,
Spending money in port
shuttle from/to cruise port from airport,
Gas to drive
I try to think of everything that will cost money. I drop it into a spreadsheet and compare different cruises that way. My spreadsheet breaks down the cost per night as well as the total cost of a cruise. A longer cruise maybe more expensive, but the cost per night is lower so the longer cruise is a better value. After I have the full costs I divide that by the amount of months before the cruise to get a cost per month I need to save. So for an April cruise I have (6 months): Sept, Nov, Dec, Jan, Feb, and March to save the money for the cruise.