re: Cruise extra costs question

There is really no way to answer that.

If you are really worried about over-spending on the cruise then I suggest buying the drink package, assuming you drink alcohol. Tip is included (although you can always tip more) in the price of the package. You also pay daily gratuities and those are a set price depending on the cruise line and exact type of room you have. You can pre-pay those if you wish, as well.

It is completely possible to get off of a cruise with a $0 balance if you plan ahead and have self control.

That goes for excursions, as well. Book ahead of time and pay them up front. If you have any OBC coming to you then put that toward any last minute excursions that you book on board or to cover any extra charges.