Hi Robyn,
I have to admit that I don't think there is a bad time of year to take a Disney Cruise! Due to the all-inclusive nature of cruise, the base price tag will appear high but when you consider all that is included it becomes more reasonable.
That being said, there are months of the year that are more affordable than others. January into early February and September tend to have the best rates. When looking at the rates, don't be afraid to book the least expensive cabin category. The majority of your time will be spent outside of your room, so if it comes down to an inside stateroom versus not cruising at all - go for the inside stateroom!
It's also worth watching for specials on the longer voyages. The Disney Dream is setting sail in 2011, and will be making all of the 3, 4 and 5-night voyages. The demand for vacations on this new ship is high, so you may find better deals (on a per-night basis) on a longer voyage on the Disney Magic. For instance, I just quickly looked up a 7-night Caribbean voyage for three guests in September 2011 - and it was $2,500. This does not include airfare, port transfers or vacation insurance, but it gives you a rough idea. It is also best to book as far in advance as possible, as the rates will rise the closer the sail date gets.
I recently explained my favorite months to sail
HERE.
Hope this helps!