Question:

How much is five shekels in modern currency? And why does the firstborn child "cost" five silver shekels, why not ten or fifty?

Answer:

Five ancient shekels amount to just about 100 grams of pure silver.

While it is customary to use silver, any movable property worth that amount (as opposed to promissory notes, paper currency, and real estate) may be used. In the US, five real Silver Dollars (such as the 1878-1921 Morgan Dollar or the 1921-1935 Peace Dollars) are used.

Many kohanim (priests) have sets of five silver dollars which they sell to others to use. If not, a visit to a coin collector or pawn shop should do the trick.

The source for an Israelite's obligation to redeem his firstborn son through giving the kohen (priest) specifically five shekels is the Book of Numbers (18:15-16): "You shall redeem the firstborn of man . . .the redemption [shall be performed] from the age of a month, according to the valuation, five shekels of silver."

Several explanations are given for the specific amount of silver shekels used for the pidyon haben. The following two are from the Talmud and Zohar, respectively:

  1. Joseph – Rachel's firstborn son – was sold by his brothers for twenty silver pieces, the equivalent of five shekels. This established that the standard "price" for a (firstborn) human is five shekels, which are given to the kohen, G‑d's representative, to redeem the child.
  2. The number five is symbolic of the Hebrew letter hei (which has the numerical value of five), which was added to Abraham's name when the time came for him to father Isaac—and the Jewish nation (see Genesis 17:5). G‑d's choice of the Jewish people as His nation, which resulted in the consecration of the firstborn and the subsequent mitzvah to redeem them, was in the merit of our forefather Abraham. We, therefore, have an allusion to Abraham at the pidyon haben.