Midrash Tanchuma (9) tells us that Moshe had difficulty understanding a machatzis hashekel. "Hakadosh Baruch Hu took a coin of fire from under His throne, showed it to Moshe, and said, יתנו זה, 'This is what they should give.'" We can explain that Moshe didn’t have difficulty understanding what a machatzis hashekel is. Moshe’s question was how was it possible for half a coin to accomplish so much? The Midrash (Tanchuma Tisa 3) states that Hakadosh Baruch Hu said to Moshe, "I swear, just as you are now standing and teaching them parashas Shekalim and you raise their heads, so will it be every year: When the nation reads this parashah before Me, it is like you are standing there, and you raise their heads."
Why would such a small donation cause ראש זקיפת, the heads of the Jewish nation to be raised?
Hashem showed Moshe a fiery coin to imply that it is the fire and the hislahavus that Klal Yisrael has when they perform this mitzvah, which is so precious to Hashem. The Chidushei HaRim zt'l was once in Germany, and he heard the Rav of a beis knesses ask in his drashah, "Why did Hashem show Moshe a אש של מטבע, a coin of fire? Hashem could have shown Moshe a silver coin! The answer is Hashem did not only show him the size and weight of the coin. Hashem showed him how the coin should be given. It should be given with a fiery hislahavus."
Towards the end of parashas Mishpatim, it states (24:17), ההר בראש אוכלת כאש 'ה כבוד מראה, "The appearance of the glory of Hashem was like a consuming fire on the mountain-top before the eyes of Bnei Yisrael."
Kedushas Levi explains, "If a person wants to know whether Hashem enjoys his service, he should see if he has a yearning and hislahavus to serve Hashem like a burning fire. If he does, this is a clear sign that Hashem enjoys his service. Heaven helps him and sends into his mind holy thoughts [of hislahavus so Hashem can enjoy his mitzvos]. This is the meaning of the pasuk, 'ה כבוד ומראה, the sign to know whether he sees Hashem's honor and that Hashem is happy with his service, the test is, אוכלת כאש, if his heart is burning with hislahavus to serve Hashem."