A connex (45' long container trailer) looks to cost around $1000-$3000 on eBay, and I think I've heard that's a typical price for one. I'd budget another $3000 to make it livable - installing drywall walls, electrical outlets, plumbing a toilet and mini-kitchenette, and a door and window.
The biggest price will be land.
Interestingly: http://containerhouse.info/how-to-buy-container-home/