A Dime a Dozen

Examples:

1. Don't feel bad about breaking up with Tom. Men like him are a dime a dozen.

2. Don't feel bad about losing your job at Wal-Mart. Jobs like that are a dime a dozen.

3. It doesn't matter what his opinion is. Opinions are a dime a dozen.

4. If your new employee doesn't work out, fire him and get a new one. Employees are a dime a dozen.

5. Lazy people like him are a dime a dozen.

What does "a dime a dozen" mean?