Go to Expedia cruises. Search all destinations, all months (unless you have a specific month in mind.) Then narrow your search by choosing just Disney Cruise Line using the tools, and sort by price. The cheapest I am currently seeing is a 3-night Magic cruise out of Port Canaveral on 2/4/16.
GET UP TO A $1000 SHIPBOARD CREDIT AND AN EXCLUSIVE GIFT!
If you make your Disney Cruise Line reservation with Dreams Unlimited Travel you’ll receive these incredible shipboard credits to spend on your cruise!