What's "I'm earning my keep" mean here?
Here's the lines from Bety in MadMen: I'm pretty sure Doctor Wayne tried to look down my neckline the other day. He sits behind me. And as far as I'm concerned, as long as men look at me that way,I'm earning my keep."
What's "earning my keep" mean here? Is it an expression in common use?
Many thanks.

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she found  a good thing ..

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It means that she's happy that  men find her attractive.

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"Your keep" is the reason to keep you.  When we say "earn (his/her/my/etc) keep", it means we provide a reason to be kept.

Yes, it is quite a common expression. Betty's meaning is that the reason she is valuable (keepable) is that she is still attractive, and she should do her best to maintain the attractiveness.

Pretty vacuous, hey? :)