The Torah, in describing the Misva of Pidyon Haben (Bamidbar 18:16), writes, "You shall make his redemption from the age of one month, with the value of silver – five Shekalim…" The Targum Onkelos translation to this verse translates the term "Shekel" as referring to the ancient "Sela" coin, and thus the Misva of Pidyon Haben requires a father to pay a Kohen five "Sela’im." The Gemara in Masechet Bechorot (50) comments that a "Sela" is equal to four "Dinarim," and the Rambam (Rabbi Moshe Maimonides, Spain-Egypt, 1135-1204), in Hilchot Erubin (1:12), writes that a "Dinar" is equal to 1.5 darham. It thus emerges that a "Sela" is equal to 6 (1.5 X 4) darhams of silver, and five "Sela’im," therefore, would be 30 (6 X 5) darhams of silver. This is, in fact, the ruling of the Shulhan Aruch (Yoreh De’a 305) – that a father must pay a Kohen 30 darhams of silver for the Misva of Pidyon Haben.

The question then becomes, how much is 30 darhams in contemporary terms?

Hacham Ovadia Yosef (Yalkut Yosef – Soba Semahot, vol. 2, p. 275) writes that after extensive historical research and analysis of Medieval coins, it was determined that the darham used in the Rambam’s time is equal to 3 grams. Accordingly, the Misva of Pidyon Haben requires paying a Kohen the value of 90 grams of pure silver. However, as Hacham Ovadia adds, the custom in Jerusalem was to use 31 darham, and thus the required is at least 93 grams of silver.

It should be noted that one does not have to use legal tender to pay the Kohen for the Misva of Pidyon Haben. Indeed, it is customary to use for Pidyon Haben five special silver coins with the relevant Pesukim from the Torah imprinted upon them. As long as one gives the Kohen at least 93 grams of silver, he has fulfilled the Misva.

Summary: The Misva of Pidyon Haben requires giving a Kohen five "Shekalim," which in today’s terms translates into 93 grams of silver.