The Sages are using the numerical value of 1/2 which is similar to 1/2 a shekel.

A person has to realize that pursuing desires is for naught since not even 1/2 ones desires come to fruition. By giving 1/2 a shekel, a person can perhaps comprehend this reality a little more. It can cause a person to reevaluate the direction his life is going in, and perhaps vital changes pertinent to his spirituality can be set into motion. The Torah is telling us to concentrate on the 1/2 shekel which represents our desires in this world. This has an impact especially when a person has to give money from his pocket. Yet as long as a person is alive changes can be made for the better. We can change the pursuit for desires into the pursuit for Mitzvahs and Torah learning. With Torah and Mitzvoth there is always a "whole" as opposed to desires which is always a 1/2 .G-d is for sure going to compensate a person for his effort in regard to Torah and Mitzvoth, yet with desires even the effort is for naught. Have a good Shabbos