eBay, and despite it having a ripped label and "Mario" written on it in ballpoint pen, the current highest bid is a whopping $5,500. That's because the cartridge is one of only 116 made for the 1990 nationwide NES tournament of the same name - and far fewer of those 116 are even accounted for.
It's a game that's popped up on auctions before: One went for $15,000 several years ago. If you think that's crazy money, we'll raise you to $33,433.30; that's how much a complete-as-complete-can-be edition of Air Raid, an Atari 2600 rarity, went for 14 months ago.
[Image Source: muresan on eBay]