Milk

• Cost of one liter of milk in plastic bottle: 2.20 (U.S. 72¢, or $2.73/gallon)
• Cost of one liter of milk in Tetra Brik: 2.19 (U.S. 72¢, or $2.73/gallon)
• Cost of one liter of milk in bag: 1.65 (U.S. 54¢, or $2.05/gallon)

Maybe you're thinking: "Oh, but the bagged milk probably comes from a dairy with brucellosis or tuberculosis." But no. Because all the milk packaging bears the same announcement:

Dairy officially free from brucellosis and tuberculosis. Fewer than 50,000 bacteria per ml.