Cruise prices are the highest in the peak summer months. They drop again starting in September except for around the holidays (Thanksgiving and Christmas). The prices have not dropped as much as in the past due to the increased cost of fuel to the cruise lines, but they have dropped. Watch the sale ads and you should be able to find something if you can travel when the kids are back in school.
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