Inflation

In my lifetime, the dollar has shrunk to be only worth $0.27. (266.11% inflation, as calculated here.) In other words, you need $3.66 to buy the same item that cost $1.00 in 1976.

While the phrase "a penny for your thoughts" originated sometime before 1546, the inflation calculator doesn't go back that far. It only goes back to 1914, since which time the equivilent value has grown to $0.21. Add in a few more centuries of inflation and the phrase should now be "a hundred dollars for your thoughts."