Cheapest that I could find several years ago was at Pep Boys. It had brackets that attatched right to the cross bars of my 'then' 03 Santa Fe. It has hinges on the drivers side and keyed locks on the passenger side. Also has prop bars to hold it open while loading.
Downside is we only used it once and it has sat in my storage space since then. Perhaps used from craigslist etc. is an option.
Another option is to purchase a cargo carrier that goes into the hitch. This is what I use. Much easier to load & unload because of height. Also no wind drag (mpg). Won't damage you vehicle with scratches, etc. Down side is it can/will interfere opening the back hatch. They do have the enclosed one that swings out of the way. Also nice when carrying something messy or smelly you don't want in the car. Here is a link to one at e-trailer.com.