There is always a discussion as to what the value of the half shekel would be nowadays. It is generally valued according to the value of silver.

Chief Rabbi Rav Yitzchal Yosef has announced that the current value of the symbolic half shekel is 24 shekel. Basically 9.6 grams of silver at a value of 2.44nis per gram comes out to 24nis. So every person would have to give 24nis for the half shekel. Rav Yosef adds that one who can and wants to be machmir can add the value of VAT to this and actually give 28nis. I am not sure why VAT would need to be added, but so be it.

Being that it is only a custom, an avreich in yeshiva should give whatever he can, however generous he wants or can be. There is no actual obligation to give this, though it is a strong custom, so someone who finds it difficult can be lenient on this. Additionally, one should be careful to not consider this the half shekel but only a symbolic donation in memory of the half shekel.

source: Kipa