Another equivalent expression in the UK is ‘two for a penny’.

I think the nearest British English equivalent might be 'cheap as chips'.

It means that it is so common it's worthless. since a dime (=10 cents) is an American coin, I expect it is only used in the US. I assume you know that a dozen is 12 of something, usually eggs, nowadays. But counting system in 12 were common in ancient times. There are vestiges of them in some European languages, and the basis for things like 12 hour clocks and 5x12=60 minutes in an hour.