Shekalim 4: The Dichotomy of Being Human
The reason the Torah stipulated a Half Shekel when the Torah just as easily could have demanded a whole Shekel was to hint to every intelligent person that he must weigh his actions in life so as to give due weight to his spiritual needs as well as to his physical needs. Ideally, he should be able to give full weight to both these requirements, i.e. a full Shekel to each. In fact, he ought to be able to use his body exclusively in the Service of God. Keeping these factors in mind, the Torah, by speaking of a half Shekel, hinted that it is aware that we suffer from this dichotomy of demands made upon us. (Rabbeinu Bachya; Exodus 30:13)
One should use the week of Shabbat Shekalim on directing his/her physical drives and needs toward the Service of God. (Machberes Avodas Hashem)