Chief Rabbi Rav Yitzchak Yosef has issued a psak as to how much money should be given for the "machatzis hashekel", as is customary before Purim.

The value of the machatzis hashekel is determined based on the value of pure silver and the exchange rate value of the dollar.

The custom, at least among Sefardim, is to give the value of 9 grams of pure silver. This was what the "half shekel" was weighted at in the time of the Beis Hamikdash. 

Based on the calculations, each gram of silver is worth 1.8nis, with 9 grams valued at at 16.5nis. It is actually 16.2nis, rounded up to 16.5nis.

Because of fluctuations in the exchange rates and the value of silver, it would be good to round up to consider the value of the "half shekel" at 20nis per person. The 20nis includes VAT, he says, though I have no idea why that is necessary.
source: Behadrei