We stayed at Circus Circus one night about 7 years ago. It is more expensive than some, but we could literally walk to (and thru) all the newer casinos on the strip quite easily. I'm not a Vegas person (nor a gambler), but the variety of structures and decor was interesting.
Circus Circus campground is really a giant parking lot with hookups. The very nice lady at the desk moved us near the pool, where we had a tiny palm tree in front of our parking space, in a postage stamp-sized patch of grass surrounded by curbing... As it was July, it was really nice to be near the pool for our daughter. Coming from weeks of camping in state and national parks, it was quite a jolt....
You can probably get a back-in space for around $50. I suspect the price is high because of the convenience factor, and because if the weather is hot, you will need to run the A/C 24/7 parked on the giant heat island of black asphalt.
Here's a link to another article that might be of help.Cheap RV Parks in Las Vegas | eHow.com