Q. How much money do you need to give for the “Half shekel”?
A. Ashkenazi Jews (from European origin) give 3 halves of the local currency where they live. Those who hold like the GRA (Rabbi Elyiahu of Vilna) can suffice with giving one such coin.
Sfardic Jews (from eastern origin) give the money equivalent of 9 grams of pure silver. Someone that can’t afford to give that can give one local coin for the mitzvah of remembering the “Half Shekel”.
The Half Shekel was originally brought in the month of Adar to the Holy Temple in Jerusalem so that every Jew would have a share in the communal daily sacrifices and all communal sacrifices across the year. The year for this reckoning was from Nisan to Nisan so in Adar the money came in to start the new counting immediately in Nisan.
Nowadays we bring a half shekel in Adar to remember that mitzvah and we hope to be able to do it again in our Holy Temple. A popular time to do this mitva is on the Fast of Esther before Purim but essentially the whole month is okay.