Orlando or Miami - check into Allegiant and/or Southwest. I think both of those cities are still on Allegiant's list - if so you can probably get the cheapest rate from them. If not, Vegas is Southwest's largest hub, with about a third of all air traffic in/out of Vegas. If your local airport also flies Southwest then that will most likely be the cheapest way.

Also, if you can stand the idea of doing a RedEye coming home (leaving Vegas late at night and getting home early morning) that will save you some bucks.