Sforno on Exodus 30:12/ספורנו על שמות ל׳:י״ב

1כי תשא..ונתנו איש כפר נפשו, the need to count human beings stems from the fact that human beings are not the same each time, i.e. ever since death was decreed upon mankind this reflects sin. Prior to sin man, who was meant to live indefinitely, would be the same at each count. The sages have said that no one dies unless he had committed a sin (Shabbat 55). It follows that mention of a head count of people is an oblique reminder of man’s sin, his guilt. This being so, it is most appropriate that at the time of such count he should pay some ransom on behalf of his soul, his life, so that he will qualify for atonement. This also explains why the poor must not give less nor the rich more, as a soul’s ransom is not calculated in terms of the amount of money offered to G’d.