Numbers 18:15-16

15Everything that breaketh open the womb of all flesh, which they present to Jehovah, of men or of beasts, shall be thine; nevertheless the firstborn of man shalt thou in any case ransom, and the firstborn of unclean beasts shalt thou ransom.

16And those that are to be ransomed from a month old shalt thou ransom, according to thy valuation, for the money of five shekels, after the shekel of the sanctuary, which is twenty gerahs.