The Plus version is hard to find these days. It seems to go for plenty more than retail ($29.99) used now. The original version used to be considered "rare", but the release of Plus pretty much plummeted its price (it's cheaper than the Plus version on eBay now). Even though you can do more hack-y stuff with the original version, I think generally people care more about the actual game.
Edited to add: There's a PSO3, but that's easy to find.