John Gill Commentary Leviticus 27:4

"And if it be a female, then thy estimation shall be thirty shekels." — Leviticus 27:4 (ASV)

And if it [be] a female
That is, of the same age, full twenty years of age, and not more than sixty:

then your estimation shall be thirty shekels ;
about three pounds ten shillings of our money, the price of a servant, (Exodus 21:32) ; the reason of this difference of estimation between a man and a woman is, because the woman is the weaker vessel, and her labour and service of less importance and worth, such as spinning, washing