The cheapest I've ever seen a large bed $30-40. I know BJs (similar to costco) has one, and I also have a great one from Target (much nice, but same price). I have two ideas for you instead of buying one if you really need to keep it under $25:
1. Do you know how to sew? If you do, I highly suggest you make you own. You can find a sturdy cheap fabric and buy a couple yards, then fold and sew with a velcro or ziper closing. Then stuff it with polyfill or foam.
2. My dogs have always loved to sleep on our old comfortors. If you have one or two comfortors you can use it as a thin bed (you could buy some at a thrift store and wash them if you don't have old ones).