If the bull gores a male or female slave, the owner must pay thirty shekels of silver to the master of the slave…-Exodus 21:32

And I said unto them, If ye think good, give me my price; and if not, forbear. So they weighed for my price thirty pieces of silver.- Zechariah 11:12

30 pieces of silver; the value of a slave.

And said unto them, What will ye give me, and I will deliver him unto you? And they covenanted with him for thirty pieces of silver.-Matthew 26:15