[quote name='outphase']Desirability of neophytes and/or collectors. Not everyone was into JRPGs when the game was big. Think of Earthbound. The game didn't sell very well, but it has garnered up a very devoted fanbase long after the game has passed its time.
Also, X sold does not mean X are still in good condition.[/QUOTE]
With 8.5 (or 9.5) million sold there are plenty in good condition. Rarity + quality are the factors that drive price. Games like Chrono Trigger are rare and of exceptional quality and expectantly have high prices. In this case, rarity is not the big factor (may be a little due to people not want to sell it), but rather the quality of the game. It's so good that it's worth paying the extra on eBay to play it. It helps that it's not made anymore, but like Chrono Trigger (my fav, but both are on par), the quality of the game plays a big factor.
If you can save $10, BUY THIS GAME. Hands down one of the best games ever made and arguably the best RPG ever made. Worth it, worth it, worth it!