Christmas, Easter and Spring Break. Just kidding, LOL!!
We used to go in mid-May (before schools let out), before Memorial Day weekend, and had good luck with short lines. We also enjoy going the first week in December...the holiday decorations are up, the crowds are lower, and the weather is still quite nice. We use to go in the fall...in mid-October...but with the ever expanding calendar for the Food and Wine Festival, more and more people seem to be there that time of year.
As for cheapest time, I really don't know. We are DVC members and just go as often as our points allow.
Best of luck planning your trip. To be honest, any time spent at WDW is a good time to be there!
- Tara