Ubiquitous; so abundant or common as to hold little or no value. In Los Angeles, waiters trying to become famous actors are a dime a dozen.That bird might be a rare sight where you come from, but around here they're a dime a dozen.
So plentiful as to be valueless. For example, Don't bother to buy one of these-they're a dime a dozen. The dime was declared the American ten-cent coin in 1786 by the Continental Congress. [First half of 1900s]