The Labor of the Plowman is of Less Value than that of the Artisan

The opportunity cost of becoming a skilled worker includes both the direct expenses as well as the foregone labor during the training period or apprenticeship. As a result, skilled workers must be paid higher wages than unskilled workers.

A LABORER’S SON, AT SEVEN to twelve years of age, begins to help his father either in keeping the herds, digging the ground, or in other sorts of country labor that require no art or skill.
If his father has him taught a trade, he loses his assistance during the time of his apprenticeship and is obligated to clothe him and to pay the expenses of his apprenticeship for many years.9 The son is thus dependent on his father and his labor brings in no advantage for several years. The [working] life of man is estimated at only 10 or 12 years, and as several are lost in learning a trade, most of which in England require seven years of apprenticeship, a plowman would never be willing to have a trade taught to his son if the artisans did not earn more than the plowmen.
Therefore, those who employ artisans or professionals must pay for their labor at a higher rate than for that of a plowman or common laborer. Their labor will necessarily be expensive in proportion to the time lost in learning the trade, and the cost and risk incurred in becoming proficient.
The professionals themselves do not make all their children learn their own trade: there would be too many of them for the needs of a city or a state and many would not find enough work. However, the work is naturally better paid than that of plowmen.