Numbers 18:16

And the redemption of them [shall be] from a month old; their valuation of five shekels—it is twenty oboli according to the holy shekel.

And those that are to be redeemed from a month old shalt thou redeem, according to thine estimation, for the money of five shekels, after the shekel of the sanctuary, which is twenty gerahs.

You shall redeem those who are to be redeemed of them from a month old, according to your estimation, for five shekels of money, according to the shekel of the sanctuary, which weighs twenty gerahs.