What is the connotative meaning if something is called “a dime a dozen? ” it is very rare. it is very common. it is very expensive. it is about the size of twelve dimes.

It is very common.
The others answers you can eliminate easily, the phrase is not ment to be taken litteraly therefore it can not be. It is about the size of twelve dimes. If you are only paying 10 cents for 12 items, this item is not very expensive. If it is not very expensive, it is most likely not very rare.

You need to attach a picture.