One thing you can try is buying a used futon mattress on Craigslist and then just cover it with a sheet. Since we rapidly go through beds over here, and have large dogs to take care of, the futon mattresses work out well and you actually get TWO for the price of ONE!
Ikea sells them for under $100.
Craigslist may even have them cheaper!
We have used these beds in small dog, indoor, obstacle courses, as well as, just being loungers.