Dan,
I'm a Sierra Designs fan myself. We backpack once or twice a summer and sometimes end up in the mountains. A couple years ago we were up at Thunder Lake in the Rocky Mountain National Park (10,500') and had rain and a little snow for almost a full day. We stayed warm and dry inside. If only we had a bathroom inside, we would of been all set. Since you won't have to worry about weight, I'm sure you can find a good tent for the price you are thinking about. Often times you pay the big bucks for the lightweight equipment, but it doesn't last as long as the heavier / cheaper stuff.
I still like your idea of loading the cat on top of a pop-up camper. Did you give up on that idea?

Jack Hoying
Fort Loramie, Ohio
Prindle 18 #1645
http://www.bright.net/~jmhoying/sailing/prindle.htm