Try this site: http://www.tonystrailers.com/ for ideas to augment your existing trailer, but I don't think you're going to find anything suitable for less money, not even if you made it yourself probably.

I didn't have the money, but I don't regret taking a loan out for my http://www.equinoxtrailers.com/trailers/ dog trailer I got from this company. I'm now noodling an additional section that rests on top of the frame, but can be removed and set next to the trailer to make it a sleeping platform for camping. Not a fully enclosed tent trailer, but more of a sleeping platform off the ground.