A penny for your thoughts
Definition 1 of 1
Pro Tip 1/3Thoughtful Prompt
Use it to kindly prompt someone to share their private thoughts or reflections in a conversation. 
You've been quiet, a penny for your thoughts?
Pro Tip 2/3Casual Expression
It's casual and usually used among friends, family, or colleagues; not in formal situations. 
We're deciding where to eat, a penny for your thoughts, Alice?
Pro Tip 3/3Friendly Interest
It expresses genuine interest in someone’s opinion or feelings in a friendly manner. 
You seemed to have an opinion during the meeting, a penny for your thoughts?