The ancient Celts measured the worth of a human life in terms of cattle. If you killed someone, the family would demand from you the precise number of cattle associated with that person's social rank. The ancient Germanic tribes had a similar concept, the weregild, which was the quantity of gold associated with the dead person's social rank.
Nowadays, I think largely due to Christian and then Enlightenment influence, we tend to think of a human life as priceless - but do we really mean that? If, in order to save one human life, we had to bankrupt every person, every business, and every nation on the planet, would that be the right choice? If not, then where do you draw the line? Would the value of a person's life be variable based on certain factors, and, if so, what would those factors be?
Human life's worth in my opinion depends on arbitrary decisions and nothing more.
In ancient times, it was a few cattle, today, well it depends. A billionaire's life is more than some random Joe normal on the street. Its the same idea as currencies, who the hell sits and decides how much the dollar is worth compared to some other world currency? Random numbers almost, assigned to a thing.
A skilled debater could easily sit here and argue a single human life is worthless. In the end, worth, like anything else, is based on somebody in a position of authority, using that authority to tell the rest of us what that worth is, and then hoping we listen and obey.