Charles Ellicott Commentary Numbers 3:47

"thou shalt take five shekels apiece by the poll; after the shekel of the sanctuary shalt thou take them (the shekel is twenty gerahs):" — Numbers 3:47 (ASV)

Thou shalt even take five shekels apiece by the poll. — It is not stated how the 273 families from whom the redemption money was exacted were chosen. However, since the law for the redemption of the firstborn by the payment of five shekels took effect from this time (Numbers 18:16), it seems probable that the money was exacted from those who had been most recently born, or perhaps the matter was decided by lot.

After the shekel of the sanctuary. — See Exodus 30:13, where the expression occurs for the first time, and the value of the shekel is stated, as in this verse.