the sheer size of Starbucks means they’d save on all varieties of milk (not just dairy) and I seriously doubt they pay 50 cents more per cup for alternative milks.

You literally have no idea though, unless you work in supply chain for Starbucks. You’re guessing. Do they do their purchasing as a single corporation from one dairy farm, I doubt it. Plus you ignore the additional hours and need for refrigerated space. There’s more to consider than just cost per unit. Also if you use less there’s a greater chance of spoilage.