If you can tolerate lactose, milk is the cheapest way to eat a good balance of high quality protein, fat, and carbs. 2400 calories for less than $3 at most grocery stores.

Then chicken is the lowest cost per calorie meat, so you're good there. May I also suggest large quantities of cheddar cheese and heavy whipping cream from Costco? Amazing value per dollar spent. Cheddar cheese (we have Tillamook at Costco out here on the west coast) is basically entirely protein and fat and then whipping cream is all fat.