For a two port cruise, you won't need very much. Porter tips (ship and airport) and a few bucks to spare. Heck I could do it for $300 and feel OK.
Ship purchases will go on your ship card. Unless you want to pay it by cash at the end of the cruise, that should be plenty.
SS Norway (12 times) '85 - '97; Radiance OS 06/01; Island Princess 11/04, Grand Princess 11/04/05, Golden Princess 7/21/06, Golden Princess 2/24/07, Explorer of the Seas 12/23/07, Island Princess 11/15/08