Now we all know the story of the Declaration of Independence that turned up at a garage sale, don't we? If we don't you can read it here: http://www.sothebys.com/liveauctions/sneak/archive/declaration.html. But read the first article - according to the Wiki, 200 copies were printed, only 24 are known to exist. Do the math.
(Note: The blog will probably be on a bit of a US theme for the next week or two as I slowly disgorge the contents of my vacation: Driving, Philadelphia, Rural New York, 100's of yard sales along the interstate and time only to stop at a few....)