When using "dozen" is it followed by "is" or "are?"
For example: A dozen is/are $50.00.
A dozen roses is/are $50.00.
Dozens of roses is/are $50.00 each.

Is there any particular rule for this?

A question like one of the above ruined my year long perfect English test average.

Thank you

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You would treat it as a normal singular noun that usually has an article with it: "a/the dozen". It also has a plural form "dozens".

If you have "a dozen" then you refer to an amount that includes 12 items - dozen here is regarded to be a -> single group, therefore it's used in singular.

If you now have several "dozens", then you have to use plural, which shows you that there are 2 or 3 or many of these dozens, and every single "dozen" again contains 12 items.
When using "a dozen" or "one dozen", it is followed by "is". The "dozen" is the subject.

Q: How much are the roses?
A: A dozen is $50.

Q: How much do you charge?
A: A dozen roses is $50.

Similarly, "dozens" is plural, and is never preceded by a number. It means indefinite small numbers, ie. Dozens of angry residents complain that their expectations have not been met. Other expressions like "half a dozen" or "a half dozen" exist too and follow the same rule as "dozen".

Other examples of one singular unit of many sub-units:

A school of fishes swims past.
An army of soldiers marches across the field.
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I would agree with you to say a dozen is singular. But not just dozen. I mean if you don't write [ a dozen] and simply write [ dozen] you would find hard to catagorize singular; it has twelve items or parts.

Unfortunately, I can't give you an example now. Nothing comes across my mind at the moment of writing.
 Pemmican's reply was promoted to an answer.
What I meant was that "dozens", the plural form, does not have a numeral in front of it. The singular one does need a/the though. Sorry if I wasn't very clear.
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Oh, I see - but you can use the plural form with a numeral, e.g.:
"2, 3, 4 dozens of eggs".