I bought a 7x4 at xmas off of ebay for $250. It had a fair bit of rust on the but I just set about it with a wire brush and then coated it in rust converter primed and the painted it. Look good as new when I was done. All up it cost me about $350 and some hard labour.
I bought a tent to sit on top for $900 off ebay as well.
http://cgi.ebay.com.au/CAMPER-TRAILE...QQcmdZViewItem
If you bide your time with ebay you can get some bargains.