Adam Clarke on Leviticus 27:4

And if it be a female - The woman, at the same age, vowed unto the Lord, might be redeemed for thirty shekels, 4. 10s. sterling, a little more than one half of the value of the man; for this obvious reason, that a woman, if employed, could not be of so much use in the service of the sanctuary as the man, and was therefore of much less value.