What does a two cents plain mean?

a two cents plain meaning in Urban Dictionary

a-two cents simple is one glass of seltzer liquid. During Great anxiety, you could get an egg lotion for twenty-five cents (seltzer, chocolate syrup, and milk), a chocolate milk (milk and chocolate syrup) for a dime, a chocolate soda (seltzer and chocolate syrup) for a nickel, or ordinary seltzer liquid for two dollars. Two dollars plain.