We live to be about 70. Or we may live to be 80, if we stay healthy. But even our best days are filled with trouble and sorrow. The years quickly pass, and we are gone.
But suppose the one who makes the special promise is too poor to pay the required amount. Then they must bring to the priest the person who will be set free. The priest will decide the right value for that person. It will be based on how much the one who makes the promise can afford.