Local stores here sell powdered milk (Cholov Stam) for about the same price as “fresh” milk, if I buy a large package. A 64-ounce package costs $16 and makes 20 quarts; that’s $3.20/gallon.
3.2 ounces of powder makes one quart of liquid milk; 1/3 cup of powder makes one cup of liquid milk.
The great advantage is the shelf life. You can keep the powder on the shelf for years, and mix it with water as you need it.