John Gill 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)

And those that are to be redeemed
Which is to be understood not of unclean beasts last mentioned, for they were redeemed with lambs and not with money; and after they were eight days and not a month old, as follows, (Exodus 13:13) (22:30) ; but of the firstborn of men:

from a month old you shall redeem, according to your estimation :
that is, take the redemption money next mentioned; and though both time and price are fixed, yet said to be according to the estimation of the priest; see (Leviticus 27:5Leviticus 27:6) ;

for the money of five shekels ;
which was about eleven or twelve shillings of our money, and is the price given for the redemption of the firstborn, when the Levites were taken in their stead, (Numbers 3:47) ;

after the shekel of the sanctuary, which [is] twenty gerahs ,
(See Gill on Numbers 3:47); of the manner of redeeming the firstborn, (See Gill on Exodus 13:15).