Charles Ellicott Commentary Genesis 37:28

"And there passed by Midianites, merchantmen; and they drew and lifted up Joseph out of the pit, and sold Joseph to the Ishmaelites for twenty pieces of silver. And they brought Joseph into Egypt." — Genesis 37:28 (ASV)

Twenty pieces of silver. —Twenty shekels of silver were computed, in Leviticus 27:5, as the average worth of a male slave under twenty. It would be about £2 10s. of our money, but silver was of far greater value then than it is now.