A penny per thought hardly seems worth it these days. However in the 1500s and earlier, it may have been a fair trade.The phrase was mentioned in 1522 by Sir Thomas More in his work “Four Last Things.” Playwright John Heywood included “a penny for your thoughts” in his catalog of proverbs published in 1546 or 1562. These are the earliest recorded uses, but the saying probably dates further back, as the penny itself has a long history.
Britain first made silver pennies around 757 A.D., and by the reign of King Edward III (1327 to 1377), the penny was the most important coin in circulation. At the time, it was worth about one-twelfth of a shilling.
The modern value is hard to pinpoint. The Straight Dope calculates a 16th-century penny would be worth $42.67 in 2001.
However, a resource for actors and writers suggests two pennies would buy a beer in the 16th century, and a French Renaissance reenactment site lists the purchasing power of a penny as equal to one loaf of bread. So a penny of More and Heywood’s era could be worth $2 to $4 in these modern times. Not a bad conversion rate, eh?