To be common and/or of very little value / available in abundance that there is no major vaue assigned to it.
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.
Books like this are a dime a dozen
Those antique dishes are pretty, but they’re a dime a dozen