I've been telling this one for years, and I think I'll be banging my head on the table for the rest of my life.
Back when I was in Tokyo in October 2009, I walked into Mandarake and saw a
Studio Half-Eye Godannar Twin Drive. The initial pricing when it came out was about 55000yen (~$550-575 at the exchange at the time). The listed price at Mandarake was ~26000yen (about $250). It was opened and used, but holy damn!
Unfortunately, I didn't have the luggage space for it to bring it back, so I unfortunately passed on it. Checking auctions these days, even broken ones go for about 60000yen (~$650), whereas intact and/or new ones can reach more, probably around 80000yen ($800-850), if not even higher.
It is currently at the top of my holiest of grails list. Hell, it's been 4+ years now, and I'm STILL regretting it. And I will probably continue to regret not grabbing it when I saw it at the at price for the rest of my life, because I'll a) probably never see one in front of me again, and b) even if I do, it'll never be at the price that I saw it at.