Purely on a flavor to price ratio, there is no better bargain than pork shoulder. You can find it for as low as $1.99/lb, and that’s all meat. Pork shoulder is incredibly flexible when it comes to flavoring and tastes fantastic thanks to it’s high fat and collagen content, which gives it a true porky flavor. Because it is a relatively tough cut that needs a long time to break down, it’s a great match for sous vide.
The only other thing that comes close, IMHO, is chicken thighs. Similar to pork, they’re very inexpensive, highly versatile and taste far better than breasts. Though they can cook pretty quickly on the stove or in the oven, so slow cooking via sous vide is really a choice, rather than a necessity.
In the old days, I would have added short ribs or oxtail in the beef family. But both have become so popular that they are no longer the bargain they once were.