Does Comcast charge by how much internet you use (such as PG E does with electricity) or do they have...?

do they have a flat monthly rate or do they charge like PG E? because last month my roommates and i payed 40 dollars for it, while it may have been for 2 months this months bill was 15 dollars which is 30 dollars over 2 months. So for the significant reduction (not complaining just curious) it seems rather odd how it all works