Kind of, but not that simple. You can pay for contracts where you commit to using x PB of traffic over some period at a heavily discounted rate or agreed amount

Kind of, but not that simple. You can pay for contracts where you commit to using x PB of traffic over some period at a heavily discounted rate or agreed amount, then pay extra over that. You can even have commitments, where if you don't use as much as you promised, you pay penalties for the difference. In edge cases, it becomes cheaper to waste some bandwidth in a given month/quarter.