I've been a collector of rare GAME BOY games for a few years now. I don't have a giant collection, but I've got some gems, one being Spud's Adventure.
However, there are "new" rare GAME BOY games that have released over the last couple of years, and you may remember that in October 2024, Green Day announced that they are "demastering" their songs to old digital tech.
One of their editions was "Welcome To Paradise" (part of the Dookie album) on a GAME BOY cartridge, alongside some pretty cool visuals and authentication from Green Day themselves.
The GAME BOY carts were won at random for $39 a ticket, and shortly after, one sold for over $2,500 on eBay.

But what I didn't know was that Green Day handed out some promotional items to "friends of Brain" (a Los Angeles-based art studio that collaborated with Green Day) beforehand to market the upcoming demastering and the launch of their "demastered" products.
One promotional item was, in fact, a promotional GAME BOY cartridge featuring Welcome To Paradise. This unnumbered promotional version comes in a different all-black casing in comparison to those given out to the 25 winners of the competition.

This promotional version also comes with a receipt, showcasing its authenticity, and that it's used for promotional purposes only by Brain x Green Day. It is unknown how many of these were made, or how many given to members of the media, but what we do know is that it could be one of the rarest GAME BOY cartridges ever made, and it's up for sale.
This cartridge is on sale for $3,000 on eBay, set as "buy it now", making it even more expensive than the numbered versions that sold last year for around $2,500.