A Dime a Dozen means cheap and common; something so much in abundance that it becomes valueless.

This phrase seems to have originated sometime in the 1800s when dime, a ten cent coin, was already in circulation and a number of different foods were being sold for the the literal price of a dime a dozen. Eggs, oranges, and other such food items were available for purchase by the dozen, and their cost was nothing more than a single dime.
Hence, we say something is “a dime a dozen” when we want to express that something is of a low value, or very common.

People who make good paintings are not a dime a dozen.
Don’t bother to buy one of these bangles; they’re a dime a dozen.