Matthew Poole and continuators on Numbers 18:16

16 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.

To wit, of men onely, not of unclean beasts last mentioned, as is manifest from the time and price of redemption here mentioned, both which agree to men, the time, Numb. 18. 16. the price Numb. 3. 46, 47. but neither agree to unclean beasts, which were to be redeemed with a sheep Exod. 13. 13. and that after it was 8 days old.

This belongs, either 1. to the foregoing clause, to this purpose, that whereas women newly delivered of a child continued in their uncleanness either a longer or a shorter time, according to the quality of the birth as it was male or female, Levit. 12. 4, 5. and the temper of the mother, the Priest was to take that time when he judged the mother to be clean, that so the mother might be purified, and the child redeemed at the same time. Or rather 2. to the following clause, to wit, the price which was to be paid, as appears by comparing this place with Lev. 27. 2, 3. And both there and here it is said to be done according to the Priests estimation, though a certain rule be added to guide his estimation and a certain price set, because the Priest was to apply that general rule to each person, and to put that price upon him.

Marginal references: Exod. 30. 13. Lev. 27. 25. chap. 3. 47. Ezek. 45. 12.