I'd have to go with some unreleased game, like Off the Wall, Dino Force, PC Cocoron, etc.
For games actually released, I'd pick Magical Chase, as there's not many other titles that I really want yet are so ridiculously overpriced. Luckily, the priciest PCE games are stuff like Kids Station and Darius Alpha, games I'd not go out of my way to pick up even if they were only $20.