I'm currently lusting over the dog trailer from Croozer. Croozer is Chariot's "low end" brand but seeing as how Chariot makes amazing trailers their low end is pretty nice. This one holds 100lbs, folds flat, and is very spacious. It even has a ring where you can attach a short leash so your pup can't jump out the minute you open the door. I believe it also has a swivel at the attachment so if you wreck your pup stays upright. You can find them online in the neighborhood of $250.

http://www.croozerdesigns.com/dog.html