Charles Ellicott Commentary Numbers 18:16

"And those that are to be redeemed of them from a month old shalt thou redeem, according to thine estimation, for the money of five shekels, after the shekel of the sanctuary (the same is twenty gerahs)." — Numbers 18:16 (ASV)

According to your estimation.— See Leviticus 18:3-7, and Notes.