I agree that price isn't the best indicator (also look at FF7 or Silent Hill on eBay!) but the crazy pricing coupled with the drought of information on the web leads me to believe that it could be a serious rarity.
Regarding Gaiaseed: it's a rare game - probably one of the top five - but I doubt it's number one. Last time I checked there was nothing about it in the rarity guide. Judging from my own experience, I think it should be either R8 or R9.
On a related note, does anyone know where to find data about the release numbers of PS games? I would like to know how many copies of "price inflated" games like Radiant Silvergun and Rakugaki Showtime were released.