Numbers 18:15-16

15 aEverything that opens the womb of all flesh, whether man or beast, which they offer to the Lord, shall be yours. Nevertheless, bthe firstborn of man you shall redeem, and the firstborn of unclean animals you shall redeem. 16And their redemption price c(at a month old you shall redeem them) you shall fix at five shekels
A  shekel was about 2/5 ounce or 11 grams
in silver, according to the shekel of the sanctuary, ewhich is twenty gerahs.
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