I've never spent an arm and a leg for any of the dresses for my formals. For my first formal I recycled my old senior prom dress because I could still fit in it...but after that it was all about Ross and Sears. I don't think it's really necessary IMO to spend more than a hundred bucks on a formal dress.
Last semester the dry cleaners lost the dress I was going to use... so luckily it was Macy's to the rescue! $59 was a great deal!