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