Something very common and not having much value Source | So plentiful as to be valueless.
Examples
Books on social media marketing are a dime a dozen.
Don’t bother to buy one of these, they’re a dime a dozen.
Low paying jobs are a dime a dozen and anyone can get one if they try, but the high paying ones are not so easy to get and require proper and relevant skills. Source: theidioms.com
Cheap watches like this have flooded the market. You can get them a dime a dozen.