A penny for your thoughts was used as an idiom in as early as 1546.
In the past pennies were minted in solid silver. It would have been quite a lot of money, especially if you were not part of the aristocracy.
On average people earned about 1-3 pence per day. So, a penny could have been a day's wages for some people.
The idiom has survived, but it is probable that the meaning has changed and the reason is inflation, or the devaluation of currency. A penny today is almost nothing. In the original context it was a day's salary.
We can conclude that in the past people cared more for other people's thoughts.
"I'll give you something of great importance and value for your thoughts."
Isn't it amazing. The words have survived, the pronunciation cannot be too different, but our values have changed and the worth of a penny is so warped with time. You just change one variable.
Perhaps this tells us that communication was even more important in the past than now, despite all of our technology.
This is a silver penny dating from the time of Henry VII.