A penny for your thoughts

Meaning: 何を考えてるの？（少し冗談っぽく言う表現）

For Example:

‣ You've been keeping silence for a long time, a penny for your thoughts!

長い間黙りこくっとるね。何考えとんか教えていた！

‣ What's with the pensive look? A penny for your thoughts.

物思いにふけってどしたん？何考えとん？

Synonym:

What's in your mind?

What are you thinking about?