Any time Moses "numbered" the Israelites (took a census, basically), they were required to pay a "ransom" price for their lives: "When you take a census of the Israelites to count them, each one must pay the Lord a ransom for his life at the time he is counted. Then no plague will come on them when you number them." (vs 12)
What was the purpose of this payment? What sort of plague would come upon the Israelites if they didn’t pay up?
Here’s what occurred to me: God was trying to teach the Israelites that their lives were not their own. In other words, He needed them to remember that they had no power within themselves to create or sustain their lives. They were totally dependent upon Him for every breath, every moment of life.
Thus, the "plague" that would come upon them if they didn’t fulfill this commandment would be the plague of self-sufficiency. They could easily fall prey to the plague of autonomy. Being required to basically "pay for your life" would be a definite reminder that your life was in another person’s hands — in this case, God’s hands.
Another interesting thing to note was that this payment was required, no matter the amount of money you had: "The rich are not to give more than a half shekel and the poor are not to give less when you make the offering to the Lord to atone for your lives." (vs 15) To me, this is further indication that the payment of the money was for the sake of the Israelites, not because God wanted money. Everyone — from the very rich to the poorest of the poor — needed to remember that God was their source of life and that they were dependent upon Him.
God is the only Source of life in the entire universe. Every day we enjoy is a gift from His gracious hand. Every breath we take is only possible because He created us and is sustaining us. There is no inherent life in anyone or anything outside of Him. Everything that lives and moves and breathes is powered by His sustaining energy. He alone is life!