Numbers 18:16

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 shalt thou redeem, according to thine 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 27:5 Leviticus 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).