And the Lord said unto Moses, Number all the firstborn of the males of the children of Israel from a month old and upward, and take the number of their names. (Numbers 3:40)
In order to substitute the Levites for them (vs. 41, 45; cf. vs. 12, 13). Since there were 273 fewer Levites, the first-born had to redeem 273 of their number at 5 shekels each (vs. 39, 43, 46-50). This was the regular redemption price of the first-born (see ch. 18:15, 16).