The Circus Circus used to have good rates, and if they were full, they would find you another hotel for the same price. You might want to give that a try.
There are tons of hotels in Las Vegas, the problem is, how close to the Convention Center do you want to be ? Obviously if you are off the strip it will be cheaper. For example, call the Station Casinos. This is where the locals hang out: Palace Station, Boulder Station, Sunset Station, Texas Station.
There is a Motel Six at Koval and Tropicana.
Super Eight at Tropicana and Industrial Road.
You might want to call Arizona Charlies. My parents stayed there once and said it was nice enough (again, not the Bellagio).
How about the Silverton ?
How about the Rio and the Gold Coast ?
If you stay off the strip, you can just drive to a strip hotel, walk through the casino to the front and have a cab take you to the convention center.