If the bull gores a male or female slave, the owner must pay thirty shekels of silver to the master of the slave, and the bull must be stoned. Exodus 21:32
So when the Midianite merchants came by, his brothers pulled Joseph up out of the cistern and sold him for twenty shekels of silver to the Ishmaelites, who took him to Egypt. Genesis 37:28
“These examples confirm that there were established prices for a life in ancient times…The price that was agreed upon for the betrayal of Jesus was the usual price of a life taken by accident or of that of a slave being sold into captivity. It would equate to about 20 days’ pay, or approximately what one would spend for a months living expenses. There could be no justification for any amount of money given in exchange for such a life, but in that time and place, it was a standard price.”
“What are you willing to give me if I hand him over to you?” So they counted out for him thirty silver coins. Matthew 26:15
Kenneth Bressett in Money of the Bible (Atlanta: Whitman, 2005) 82.