So you are asking what your sign and sail account balance may be at the end of your cruise assuming no shore excursions and the cruise itself already being paid for?
That's hard to say because it depends on how much drinking and gift buying at the on board shops you do. But you will be charged the first day for $10 per person per day for tips. so that is $100 (5 days) then I would budget about $7 per drink ($4 per beer) you think you will have. So at 3 drinks per day for both of you, you are looking at another $210. Then pictures come in play. They take your picture at least 3 times a day it seems, so those can add up too if you insist on buying them all (typically a 8x10 is about $15)
The only thing you have to add on would be the tips. The rest is up to you. But for us on a typical 7 day cruise with a family of four and a wife that buys nearly every picutre they take, we typically (with exursions) have a $2000 bill at the end of the cruise. With about $800 to $1000 of that being excusrions.
Hope that helps.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise, Magic 1/14
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013