I'll admit up front that this one is not exactly an advergame, but it's close enough for Internet work.
In 1983 Coca Cola commissioned Atari to create a cartridge for the company's annual sales convention. The result was Pepsi Invaders, a Space Invaders variant in which the aliens were replaced by letters spelling out PEPSI. The production run was 125, one for each of the attendees of Coke's sales convention that year. Sales executives also received an Atari 2600 console along with the game. The assumption must have been that shooting down Pepsi at night might inspire more fervent sales efforts during the day.The very small production run for the game probably makes it the rarest title for the Atari VCS. Since Coke sales executives didn't know the difference, the title is also very hard to find - it would have found its way into attics and closets unnoticed. To make matters worse, the cartridge has no label.
The last time I saw one sell it went for $US1,800, although it looks like we just missed one a week ago on Ebay, which was unable to get its $US975 asking price.
Pepsi Invaders [Atari Age]