This is an expression that is mostly used in informal contexts. When we say something is ‘dime a dozen’ , It means It is very uncommon ; It is abundant , and is of very little value.
Background– In U.S. a ‘dime’ is a ten-cent coin. If you can buy a dozen items for ten-cent, it means that the things you are buying are very cheap indeed. This example became popular in 1930s.
Examples– 1. I can easily find someone to replace Joy. Workers like him are dime a dozen.
2. In our country , crooked politicians are dime a dozen.
3. Sarojini Nagar is dime a dozen market place in Delhi.