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Example sentences from the web:

• A: What do you think of global warming?
  B: I don’t know.
  A: Penny for your thoughts?
  B: Well, if you want to know, I feel like people aren’t doing enough to stop it!
• Noticing his friend was sad, Joe asked, “A penny for your thoughts?”
• A: What do you think about Lisa? Penny for your thoughts?
  B: People don’t like Lisa very much because she is always giving her opinion, even if people aren’t asking for it.

A video on this idiom by Englishcafe: