Numbers 3

The Levites would replace the firstborn as the people who would be serving God and the priests around the tabernacle. The Levites proved they were the right people, despite being the 3rd born son, by their actions after the golden calf.

The 273 firstborn over the 22,000 redeemed firstborn sons still had to be redeemed, so 5 shekels per son was the price.