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Common, 120 for a dollar it seems? (1,4,1,5)

Ross

I believe the answer is:

a dime a dozen

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'common' is the definition.
(I've seen this in another clue)

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for a dime a dozen that I've seen before include "Cheap (for Americans)" , "Very common; of little value" .)

I've seen this clue in The Telegraph.
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