Rental car companies typically charge 3-4 times the street price per gallon to fill up your gas car. They don't like people returning cars low on gas as then they have to pay someone to drive to a local gas station. Having fuel pumps means expensive EPA permits (or whatever agency in your country ensures they don't pollute) and a lot of safety systems - better to let a real gas station handle that.. So I would expect their prices are unreasonable just because of tradition.

Of course electricity is something they already have, and in theory it should be a lot easier for them to handle chargers. However it is still new to them and they don't have it yet.