Numbers 18:15-16

1512Every first issue of the womb of all flesh, whether man or animal, which they offer to the Lord, shall be yours; nevertheless the firstborn of man you shall surely redeem, and the firstborn of unclean animals you shall redeem.

16As to their redemption price, from a month old you shall redeem them, by your valuation, five 3shekels in silver, according to the 4shekel of the sanctuary, which is twenty gerahs.