$130, w/ a PSU.
It depends - if you're on a very tight budget, it's obviously the easiest place to cut corners. (You can find ATX towers for $30-$40, free shipping, all the time. Sometimes even less. A far cry from the $100+ that a lot of people spend on their cases.)
But if you have a little extra money, it IS the part that you actually look at every day, so finding one you like aesthetically isn't a terrible idea. (Going from bargain basement $30 cases to, say, $100, gets you a LOT of midrange options. Amortized over the life of the case/computer, that's probably <$1/month.)
And if you intend to walk the upgrade treadmill, upgrading individual components every few months instead of doing a complete rebuild every few years, spending some extra money on a case with tool-less assembly, rolled edges, etc., can save you some aggravation in the long run.
At the high end of the cost spectrum, the overclocking and gaming enthusiast community have their own needs, which determine things like internal volume, layout, number of fan/radiator mounts, etc.. They're willing to pay the money, so the manufacturers charge it.