This is such an individual issue. First, if you are talking about cash, you can charge everything at WDW and on board to your KTTW card. In that case, you will want a small amount of cash to tip the baggage porters and some "just in case" for the stop in Nassau.
If you are asking how much is a reasonable amount to spend on board the ship....you can go from nearly nothing to "the sky's the limit."
Photos--$10 each for 6X8 size, $20 for 8X10. Way to save--in most cases there is a CM right there who will take a photo with your camera
Spa--you are right that the spas at WDW are less than on board. I prefer to do a spa day at home (if I have time) before I leave and save my ship time for other things. My favorite on board is the rainforest which costs $15 per day. THere is also an umlimited use pass for most cruises.
Gift shop --as little or much as you want. THey sell everything from pens to diamonds on board. Savings--If you have a car, there is a Disney outlet a few miles from WDW. SOmetimes they have a great selection, sometimes nothing.
Palo--$10 per person plus any additional tip. Just do it!
Excursions--as little or much as you want. You can save by booking things on your own instead of thru
DCL.
WDW meals...if you are doing a package, you are not on the
Disney Dining Plan. Is it possible to add this to your deal? It is a huge savings if it can be done with whatever your deal is.
If you are on the land/sea, you will have a 7 day pass and will only use 4 days of it....You can make it "never expire" for an additional $55, and will have tickets to play another day.
Alcohol...again, whatever you desire. Drinks are about $5-6 each. There is an automatic 15% gratuity.
Tips--there are whole threads on this. Recommended tips run about $11 per cruiser per day. You tip your server, assistant server, head server, and room steward. Many people tip a little more when they realize how good the service is.
These are the main places to drop money. We have done a 7 day cruise for as little as $250 + tips over the cruise/transportation,e tc....and as much as $2000. In fairness, the $2000 included some nice jewelry.