Leviticus 27:1-8

The LORD spoke to Moses: “Speak to the Israelites and tell them: When someone makes a special vow  to the LORD that involves the assessment of people, if the assessment concerns a male from twenty to sixty years old, your assessment is fifty silver shekels  measured by the standard sanctuary shekel.  If the person is a female, your assessment is thirty shekels.  If the person is from five to twenty years old, your assessment for a male is twenty shekels  and for a female ten shekels. If the person is from one month to five years old, your assessment for a male is five silver shekels,  and for a female your assessment is three shekels of silver. If the person is sixty years or more, your assessment is fifteen shekels for a male and ten shekels for a female. But if one is too poor to pay the assessment, he is to present the person before the priest and the priest will set a value for him. The priest will set a value for him according to what the one making the vow can afford.