When you return a ICE car partially empty someone has to either bring gas to the vehicle or drive the vehicle to a pump.
Even though this is probably on-site it is very labor intensive and most rental places are pretty low staff. I could see future EV ones being entirely self service.
Anyway an EV can just be charged where you park assuming they wire each spot with an EV charger which I think makes sense since doing an n+1 charger is probably not much more.
I think the you will just pay whatever the normal charger electricity price would be.