Adam Clarke on Genesis 37:28

For twenty pieces of silver - In the Anglo-Saxon it is thirty pence. This, I think, is the first instance on record of selling a man for a slave; but the practice certainly did not commence now, it had doubtless been in use long before. Instead of pieces, which our translators supply, the Persian has miskal, which was probably intended to signify a shekel; and if shekels be intended, taking them at three shillings each, Joseph was sold for about three pounds sterling. I have known a whole cargo of slaves, amounting to eight hundred and thirteen, bought by a slave captain in Bonny river, in Africa, on an average, for six pounds each; and this payment was made in guns, gunpowder, and trinkets! As there were only nine of the brethren present, and they sold Joseph for twenty shekels, each had more than two shekels as his share in this most infamous transaction.