I'm not sure how it's calculated, but I know that the fee is added daily. It's not a one time fee. For example, if you reserve a car for a day it might cost $150. But if you reserve it for a month it will likely cost $3000+. But for the same car not going one way it may only cist $600 for a month. I hope this makes sense.

In other words, if you are going one way but intend to have a rental for longer than a few days, it's probably better to rent the one way vehicle for as short of time as possible and switch to another rental when arriving at your destination.