For all the amazing games Valve has produced (and they are amazing, make no mistake), they also kicked off the lootbox trend with microtransactions in Team Fortress 2 and Counter-Strike. Even their masterpiece Portal 2 tried a similar system, although much of the discourse surrounding that game is rightfully about the masterful single-player campaign instead.
It's only fitting, then, that this same trend would reach its apotheosis in a Valve game: an AK-47 skin in Counter-Strike: Global Offensive has sold for $400,000, making it the most expensive in-game item, official or otherwise, ever. There have been bigger total transactions, also from Counter-Strike, but this easily takes the crown as the biggest pricetag attached to a single item. It seems to me that this isn't too far removed from the recent NFT craze, with speculators forking over massive amounts of money for digital assets that may or may not appreciate in value. At least CS:GO transactions don't use enough energy to power a small country.