Albert Barnes Commentary Leviticus 27:2-3

"Speak unto the children of Israel, and say unto them, When a man shall accomplish a vow, the persons shall be for Jehovah by thy estimation. And thy estimation shall be of the male from twenty years old even unto sixty years old, even thy estimation shall be fifty shekels of silver, after the shekel of the sanctuary." — Leviticus 27:2-3 (ASV)

Rather, When a man makes a special vow which concerns thy valuation of persons to Yahweh, if thy estimation shall be of the male, etc. The expression “thy estimation” is addressed either to Moses or to the priest (Leviticus 27:12): it denoted a legal valuation. The vow of a person was perhaps most frequently made in cases of illness or danger, under the impulse of religious feeling, either in the way of thankfulness for blessings received, or of supplication for something desired. A man might dedicate himself, his wife, his child, or his bondservant. This might have been an old custom; but the Law ordained that he who had taken such a vow should pay a sum of money to the sanctuary, determined according to the age and sex of the person.