And those that are to be redeemed from a month old shalt thou redeem, according to thine estimation, for the money of five shekels, after the shekel of the sanctuary, which is twenty gerahs. (Numbers 18:16)
The price set when the first-born were exchanged for the Levites (ch. 3:46, 47). Jews today have a ceremony derived from this redemption, when a firstborn son is a month old.