Ready made Chicken Tractors-Which one is the sturdiest?

[Chicnsrthebest]

I've been looking at the ready made chicken tractors, and a lot of them are not very sturdy and don't look like they will last long.
Does anyone out there know of one around $500 (or less) that they would recommend as being a nice, sturdy one for a few chickens?

I know building my own would be more cost efficient and sturdy, but I'm looking for your votes on ready made tractors. Any takers?