My ex-husband and I drove to WDW from North Carolina once, about 10 years ago. We didn't have much for the vacation at that point, so we stayed at the campground for something like $30 a night. We still spent money on the tickets and food, but it's hard to find a cheaper stay than $30 a night!
On the other hand, it was pretty damp and sweaty in the tent at night, and after walking all day at the parks, the last thing you want to do is try to sleep on an air mattress! If you had an RV, that would probably be a better choice (although probably not as cheap)!