A penny a day - The coin here referred to was a Roman coin, equal in value, at different periods, to 15 cents or 17 cents (7 1/2 d. to 8 1/2 d.) (circa 1880's). The original denotes the Roman denarius δηνάριον dēnarion, a silver coin, which was originally equivalent to ten ases (a brass Roman coin), from which it gets its name. The consular denarius bore on one side a head of Rome, and an X or a star, to denote the value in ases, and a chariot with either two or four horses. At a later period the casts of different deities were on the obverse, and these were finally superseded by the heads of the Caesars.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.
It was probably at that time the price of a day's labor. See Tobit 5:14. This was the common wages of a Roman soldier. In England, before the discovery of the mines of gold and silver in South America, and consequently before money was plenty, the price of labor was about in proportion. In 1351 the price of labor was regulated by law, and was a penny a day; but provisions were of course proportionally cheap, and the avails of a man's labor in articles of food were nearly as much as they are now.

A penny - A Roman coin, as noted before, Matthew 18:28, worth about seven-pence halfpenny or seven-pence three farthings of our money, and equal to the Greek drachma. This appears to have been the ordinary price of a day's labor at that time. See Tobit 5:14. In 1351 the price of labor was regulated in this country by parliament; and it is remarkable that "corn-weeders and hay-makers, without meat, drink, or other courtesy demanded," were to have one penny per day! In 1314 the pay of a chaplain to the Scotch bishops, who were then prisoners in England, was three halfpence per day. See Fleetwood's Chronicon Precios, p. 123, 129. This was miserable wages, though things at that time were so cheap that twenty-four eggs were sold for a penny, p. 72; a pair of shoes for four-pence, p. 71; a fat goose for two-pence halfpenny, p. 72; a hen for a penny, p. 72; eight bushels of wheat for two shillings, and a fat ox for six shillings and eight-pence! Ibid. In 1336, wheat per quarter, 2s.; a fat sheep 6d.; fat goose, 2d. and a pig, 1d., p. 75.

And when he had (a) agreed with the labourers for a penny a day, he sent them into his vineyard.
(a) Literally, "fell in time": it is a kind of speech taken from poetry.

And when he had agreed with the labourers for a penny a day,.... These labourers were of that sort that were called , "hired for a day"; concerning whom is the following rule (q):
"he that is hired for a day, may demand it all the night; and he that is hired for a night may demand it all the day: he that is hired for hours, may demand it all the night, and all the day; he that is hired for a week, he that is hired for a month, he that is hired for a year, he that is hired for seven, if he goes out in the day, may demand all the day; and if he goes out in the night, he may demand it all the night, and all the day.''
And the wages of a day were usually "a penny"; which, if understood of a Roman penny, was seven pence halfpenny of our money. One of their canons runs thus (r):
"he that hires a labourer in the winter, to work with him in the summer, , "for a penny every day", and he gives him his hire; and, lo! his hire is alike to that in the winter, a "sela" every day, this is forbidden; because it looks as if he chose that time to lessen his wages; but if he says to him, work with me from this day, to such a time, "for a penny every day", though his hire is the same, a "sela" every day, this is lawful.''
By the penny a day agreed for with the labourers, may be meant external privileges; or the free promise made, whether to ministers, or private believers, of a sufficient supply of grace daily, that as their day is, their strength shall be; together with that of eternal life and happiness at last.
He sent them into his vineyard; to labour there: for none have any business there, but such who are called and sent by the owner of it; and where sons are sent, and work, as well as servants; see Matthew 21:28.
(q) Misna Bava Metzia, c. 9. sect. 11. Maimen Hilch. Shericut, c. 11. sect. 2. (r) Maimon. Hilch. Milvah Ulavah, c. 7. sect. 12. Vid. T. Bab. Bava Bathra, fol. 86. 2. & 87. 1. & Gloss. in ib.

And when he had agreed with the labourers for a penny--a usual day's hire.
he sent them into his vineyard.

Agreed with the labourers for a penny a day. A denarius, about sixteen cents, the usual full price of a day's labor at that time. It would buy then more than a dollar will now.

The Roman penny was about seven pence halfpenny. [About thirteen and three quarter cents, American.] This was then the usual price of a day's labour.