penny for your thoughts, a
What are you thinking about? For example, You've been awfully quiet—a penny for your thoughts. This expression dates from the 1500s and was in John Heywood's 1546 collection of proverbs.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.