Paragraph 7: How much can you sell a 12 year for?

From the text:

I am assured by our merchants that a boy or a girl before twelve years old is no salable commodity; and even when they come to this age they
will not yield above three pounds, or three pounds and half a crown at most on the Exchange; which cannot turn to account either to the parents or the kingdom, the charge of nutriment and rags having been at least four times that value.