Periods when listing currency are usually used to denote cents, or fractions out of 100 of 1 single unit of currency (so you would never have more than two decimal places [in a base ten number system {hence 'decimal'}]).
Commas are used to denote jumps in grand amplitude, ie, from hundreds, to thousands, from hundreds of thousands to millions, from hundreds of millions to billions, and et cetera. It is not strictly necessary to do this, but it often is done to make large numbers easier to read by humans. It does also mean though you would not (properly) see someone use a comma for only two decimal places.