John Calvin Commentary Numbers 3:45

"Take the Levites instead of all the first-born among the children of Israel, and the cattle of the Levites instead of their cattle; and the Levites shall be mine: I am Jehovah." — Numbers 3:45 (ASV)

Take the Levites instead of all the first-born. The compensation I have spoken of follows; for, since God's complete portion was not found in the tribe of Levi, it had to be supplied from elsewhere. Since the Levites, infants as well as men, were two hundred and seventy-three fewer than the firstborn of the twelve tribes of Israel, God required that five shekels of the sanctuary be paid for every head. We have seen elsewhere that the shekel of the sanctuary was double, amounting to two ordinary ones.