But I'm pretty sure there is no mandated federal minimum retail price, although some individual states (Maine, California, Pennsylvania, probably others) have state-specific laws regarding retail pricing. Are you maybe confusing a state law where you live with national policy? Or am I out-of-date?
I am surprised by how inexpensive compared to local brands Walmart's Great Value milk is where I shop in Vermont. My guess was that this is because the local brands pay their producers at the higher North-Eastern rate, whereas Walmart sources their milk from the mid-West and can pay a much lower wholesale price.
The people who make money on dairy farms are the ones who chemical companies and pharmaceutical companies. The cost of raising milch cows eats away at margins. Dairy farmers survive because of subsidies until they have to keep producing more and more ..they stretch and stretch and then they collapse. Tragic.