I am quite a few years late to this but last night picked up an Xbox Series X (thanks marketplace) with the sole intention of playing RDR before playing RDR2, and then today saw that Red Dead Redemption was on sale on the Microsoft store. Can now play it in upscaled 4K goodness, or 1440p on the Series S.
Technically perhaps not a deal for those with a Series X as you can just buy a used 360 version, but for those with Series S this is about as cheap as it gets to own the game.
A chunk of other games on sale as well: https://www.xbox.com/en-au/games/all-games?cat=onsale
But don't forget to check game pass first etc
Played through this via BC on my Series X last year. Game still holds up so well.