The idiom refers to dimes, which is a small unit of American currency. While dimes were first minted in 1726, in the 1800s many goods such as eggs or apples and oranges were advertised as costing one dime for a dozen units. The phrase began to be a catchphrase for a good deal or good value for money, where the buyer could get a bargain (getting a dozen of an item for a small amount of money). Today, it has evolved to mean that an item is almost worthless because of how common and easily available it is.
Examples
Superhero films are a dime a dozen nowadays.
Everyone seems to be buying an iphone these days. It’s like a dime a dozen.
