The phrase a dime a dozen refers to something very plentiful, common, and therefore, inexpensive. A dime is a unit of U.S. currency that is one tenth of a dollar, or ten cents. The dime was first minted in 1796. In the 1800s, many goods such as eggs or apples were advertised to cost a dime a dozen in the United States.

A dime a dozen - Grammarist

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