The Jewish religious leaders wanted arrest Jesus and have Him tried for blasphemy and treason. But they needed to take him into custody in a secluded place because of his popularity with the common people. They paid Judas to let them know when He could be arrested discreetly and under the cover of darkness.
The exact identity of the thirty silver coins that they paid Judas for providing this information is not known. They may have been the common silver coins issued by the Romans New Testament times that were approximately equal in value to the Old Testament shekel (Genesis 23:16). The value of a slave was placed at thirty shekels in Old Testament times (Exodus 21:32).
MATTHEW 26:15 – They [the religious leaders] covenanted with him [Judas] for thirty pieces of silver [thirty silver coins, NIV]