This they shall give... half a shekel (30:13)
The mitzvah of the half-shekel is that each should contribute a coin that [is valued at] half of the dominant coin of that time. If the prevailing coin is takal, they should give a half-takal; if it is a selah, they should give a half-selah; if it is a darcon, they should give a half-darcon.
(Midrash HaGadol)
Why not a complete coin? To teach us that no man is a complete entity unto himself. Only by joining with another can a person become whole thing.
(The Chassidic Masters)
G‑d took a coin of fire from under His throne of glory and showed it to Moses, saying: "Such as this they shall give."
(Midrash Tanchumah)
Moses could not understand: How could a mere coin serve a person as "a ransom for his soul to G‑d"? G‑d answered him by showing him a "coin of fire." G‑d was saying: When a person performs even a modest act of charity with the fire of passion and enthusiasm, he is giving a piece of his soul...
(The Rebbe of Kotzk)
Money is fire. Like fire it can destroy and annihilate, or illuminate and warm, depending on how it is used.
(Rabbi Elimelech of Lizensk)
The rich shall not give more, and the poor shall not give less (30:15)
People differ in their intellect, character, talent and sensitivity. But all are equal in the very basis of their bond with G‑d: their intrinsic commitment to Him. So while every man contributed to the making of the various components of the Sanctuary in accordance with their individual capacity, all gave equally of the silver of which its foundation was made. As regards the foundation of the relationship between man and G‑d, the rich man cannot give more, and the pauper cannot give less.
(The Lubavitcher Rebbe)