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 phrase that began as a way to tout good value for the money evolved into a phrase that means something nearly worthless by virtue of its commonness and easy availability.

What does dime a dozen really mean?

So plentiful as to be valueless. For example, Don't bother to buy one of these—they're a dime a dozen.

Is dime a dozen an insult?

“Dime a dozen” origins

Over time, the idiom evolved to mean the opposite. Instead of something being a good deal, it became a phrase to describe something that's not valuable and easily available.