From “Obama’s Jobs Search” (by Peter Baker), NYT Magazine of January 23, p. 41, a quotation from supermarket cashier Brittany Burton:
I had a hard time. I think I applied at 19 places when I was still counting, and I hadn’t found anything, so, yeah, it was pretty bad. Especially with the big-box places, because they have so many employees already, you’re not really worth much. You’re a dime a dozen.
That’s the idiom (be) a dime a dozen ‘(be) common, easy to find, cheap, almost worthless’ (ADAD for short), here with the subject you (sg.). I realized that ADAD with many singular subjects struck me, at least initially, as odd:
I’m a dime a dozen.
Brittany is a dime a dozen.
My hat is a dime a dozen.
Such examples can be interpreted if their subjects are understood as denoting types or kinds rather than individuals: ‘someone/something like Subject, people/things like Subject’ (with the relevant sort of likeness determined from context). And so it is in the sentence from Brittany Burton, which has generic you as its subject.
Googling nets tons of ADAD examples with plural subjects: those, good ideas, eclipses here, wet rocky planets, overachievers, million-dollar homes (!), billionaires, true friends, bikini babes, online phone cards, bad teachers, people with your skills, etc. And a great number with indefinite-generic subjects: something, a thing, someone like you, a good idea, a good singer, a good short story, etc.
There are some ADAD examples with proper names as subjects, but the proper names are understood as referring to types:
Jessica Burciaga Is A Dime A Dozen (here)
meaning that women like her are commonplace, that there’s nothing special about her. And then there’s the line from Death of a Salesman:
I’m not a dime a dozen! I am Willy Loman!
with “I’m not a dime a dozen” meaning that people like me are not common; I am special, even unique.