Yes, it's about how collectable it is. (I don't like throwing around the term "rare" for everything that fetches a pretty penny since, for example, Final Fantasy 7 isn't rare; it's simply highly sought after.) I recently sold a sealed copy of the Elder Scrolls IV: Oblivion: Game of the Year Edition strategy guide for $150. Yes, that's insanely high, generally speaking, for a guide and yes, detailed walkthroughs can be found online, but collectors will pay big money for original video game items. Another example is the Splatterhouse games, which still fetch a good amount even though all of them can be played on the current gen remake.
Anyway, I have auctions on eBay ending soon: http://www.ebay.com/sch/digiskai/m.html?_nkw=&_armrs=1&_from=&_ipg=&_trksid=p3686.