Fourlakes- Like the others have said, trying to get on a ruise for the Christmas break is NOT going to be cheap. If at all possible, go after (the week of the 6th of Jan.) All the big cruiseline are running specials for that week.
Of course you may not be able to go at that time, but if you can, that is your best bet. The cruise lines are experts at knowing what dates are good travel dates for people and always bump up the fares for those dates.
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
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013