The Mitzvah of Pidyon ha'Ben is the same in all times. The amount is not 5 silver dollars. Rather, one must give the value of 5 Sela'im coins of the Torah, which are pure silver coins with a known weight. So, if the price per gram of silver goes up, the Mitzvah becomes more expensive, and vice versa. One may also give items equal to this value. However, people like to form coins of silver with the proper weight and use them, for that gives them the feeling of giving five Shekalim, and it also then certainly has the proper value, but there is no requirement to do this.

All the best,

Reuven Weiner