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
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