"Everyone included in the census must give a half shekel. This shall be by the sanctuary standard, where a shekel is 20 gerahs. It is half of such a shekel that must be given as an offering to God."
A man's body is a hint to this shekel. The word “shekel” is closely related to “mishkal,” meaning “weight” and to the weighing scales. Man's two hands are the two cups of the scales, and his arms are the scales' beam. Since a hand has five fingers, it is represented by the letter hey (ה). We thus have two letters hey, the letter vav (ו) as the beam, and the letter yud(י) as the ideal holy shekel. All together they create the name of God yud-hei-vav-hei (יהוה).
The first hey in the name of God is the “rich hey”, and its numerical value is five, but each of these units is ten (since this is “Ima Ilaa,” or “Supernal Mother”, one of the highest spiritual entities in the four spiritual worlds), and thus the value of this first hey is 10 * 5 = 50. The second hey, the completion of God's name, is represented by the Jewish people. Since this physical world is spiritually impoverished in comparison to the the upper worlds and to its true potential, this letter hey is called the “poor hey.”
Therefore the “rich should not give more than half a shekel.” The rich is the righteous, his intention is to unify the name of God, and he wants to connect the lower and the upper hey. And yet, he cannot bring more than ten Sefirot, because (as Sefer Yetzirah points out), there are ten ways of God's interaction with the world, and not more. The poor (one who is poor in spiritual knowledge and who acts within the confines of the physical world), cannot bring less than half a shekel: his “ten” are for the poor, worldly hey, yet he brings them all.
That is why at the Passover Seder we talk about “This is the bread for the poor.” The poor usually asks at the door, and the letter hey has the shape of a door (ה). “All who need should come and eat” - because it is the spiritually poor who come to the “rich table” of the Seder.
Art: Balancing the scales (after) Gabriel Metsu