Check with your cruise line for guidance on their daily gratuity charge per passenger per day. Its usually $11-12/day per passenger. The gratuities are parsed out to the dining crew and the cabin/house keeping crew that service you during the cruise.

Its good to know the gratuities charge because if you are in the shopping phase and have a choice between free pre paid gratuities or an OBC (On Board Credit $$), you can determine which is more beneficial based on $$ value. You can always ask your travel agent for these amenities (free gratuities or OBC). Here are some comparisons of deals with free gratuities or OBCs:
http://www.cruisedeals.com/deals/cru...s_7_nights.asp