leviticus 27:2 Speak unto the children of Israel, and say unto them, When a man shall make a singular vow, the persons shall be for the LORD by thy estimation. - Free Bible Online

      2-8. When a man shall make a singular vow, &c.--Persons have, at all times and in all places, been accustomed to present votive offerings, either from gratitude for benefits received, or in the event of deliverance from apprehended evil. And Moses was empowered, by divine authority, to prescribe the conditions of this voluntary duty.
      the persons shall be for the Lord, &c.--better rendered thus:--"According to thy estimation, the persons shall be for the Lord." Persons might consecrate themselves or their children to the divine service, in some inferior or servile kind of work about the sanctuary (1Sa 3:1). In the event of any change, the persons so devoted had the privilege in their power of redeeming themselves; and this chapter specifies the amount of the redemption money, which the priest had the discretionary power of reducing, as circumstances might seem to require. Those of mature age, between twenty and sixty, being capable of the greatest service, were rated highest; young people, from five till twenty, less, because not so serviceable; infants, though devotable by their parents before birth (1Sa 1:11), could not be offered nor redeemed till a month after birth; old people were valued below the young, but above children; and the poor--in no case freed from payment, in order to prevent the rash formation of vows--were rated according to their means.

JFB.