Numbers 18:16

18:16 And those that must be redeemed you are to redeem when they are a month old, according to your estimation, for five shekels of silver according to the sanctuary shekel (which is twenty gerahs).

Leviticus 27:6

27:6 If the person is one month old up to five years old, the conversion value of the male is five shekels of silver, and for the female the conversion value is three shekels of silver.