Albert Barnes Commentary Matthew 20:2

"And when he had agreed with the laborers for a shilling a day, he sent them into his vineyard." — Matthew 20:2 (ASV)

A penny a day. The coin referred to here was a Roman coin, equal in value to about 14 cents, [approximately 7 pence]. The original text denotes the Roman denarius (dhnariou, a silver coin), which was originally equivalent to ten asses (a Roman brass coin), from which it derived its name.

The consular denarius bore on one side a head of Rome, and an X or a star to denote its value in asses, and a chariot with either two or four horses. At a later period, the images of different deities were on the obverse, and these were finally superseded by the heads of the Caesars. Many specimens of this coin have been preserved. The accompanying illustrations show the usual appearance of these coins.

This amount was likely the price of a day's labor at that time . It was also the common wage for a Roman soldier.

In England, before the discovery of the gold and silver mines of South America and consequently before money became plentiful, the price of labor was at a similar proportional rate. For instance, in 1351, the price of labor was regulated by law at a penny a day. However, provisions were, of course, proportionally inexpensive, and the purchasing power of a man's labor in terms of food was nearly as much as it is today.