Bechorot 50 - Shekel, Selah, and Other Currencies
The amount given to a Kohen to redeem the firstborn son is " five shekels by the sanctuary standard, where the shekel is 20 gerah ." In the times of the Second Temple the names of the coins changed. Since a common payment was a half-shekel, donated yearly by each Jew to the Temple for atonement sacrifices, this half-shekel was called a "shekel," and the full shekel was instead called a "selah". That is why our ruling was formulated in terms of "shekel by the sanctuary standard," and not just a "shekel."
The same "shekel of the sanctuary standard" was used for other payments prescribed by the Torah: the fifty-shekel payment of the rapist and of the seducer of a virgin young girl , and a one hundred-shekel payment of the defamer of his new bride .
These payments can be made with money or with goods, except for the half-shekel collected yearly for sacrifices. They are paid in Tyrian mintage, which was pure silver and eight times more valuable than the "provincial" coins used, for example, for a two hundred-zuz payment of a Ketubah given in the case of divorce. A sanctuary shekel is .8 oz. silver .
Art: Auguste Charpentier