This is obviously an idiom “coined” originally in America (sorry for the pun, I couldn’t resist) as it’s the only country to call a coin a dime.
“A dime dozen” means something that is cheap, plentiful, abundant, or common. Some examples are:
“That baseball card isn’t rare. They’re a dime a dozen on E-bay.”
“Cheesy romance novels are a dime a dozen.”
The birth of this idiom is startlingly recent. In 1976, the US Continental Congress officially named the 10-cent coin, the “dime.” The thin little coin grew in popularity so fast that millions upon millions were minted. They were everywhere! Dimes were so plentiful, everyone had some, and thus the phrase “a dime a dozen” was born.