She’s not worth much! (5)

I believe the answer is:

penny

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'worth much' is the definition.
The answer and definition can be both quantities as well as being singular nouns.
Maybe there's a link between them I don't understand?

'she's not' is the wordplay.
'she' becomes 'pen'.
'not' becomes 'ny' (I can't justify this - if you can you should believe this answer much more).
'pen'+'ny'='PENNY'

Can you help me to learn more?

(Other definitions for penny that I've seen before include "Girl" , "One hundredth of a pound" , "Little bit of 18" , "Bronze coin" , "Denarius" .)

I've seen this clue in The Mirror.

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