I think some products that got really low print runs will go for big bucks to the handfull of people that want them, and be of no value to anyone else. Like the new Temple of the Frog
3E update module that came out last Gen Con from Dave Arneson's company (not the
WOTC online sequel adventure). There may be 100 or less copies in print, and I have yet to find anyone that actually owns a copy. If one ever pops up on Ebay, there will probably two or three people that will bid it up pretty high.