Numbers 18:15-16

15Every thing that openeth the womb in all flesh, which they bring to the LORD, whether of men or beasts, shall be thine: nevertheless the firstborn of man shalt thou surely redeem, and the firstling of unclean beasts shalt thou redeem. 16And those that are to be redeemed from a month old shalt thou redeem, according to thy estimation, for the money of five shekels, after the shekel of the sanctuary, which is twenty gerahs.