If you pick up a car in one country and drop off in another, there will always be a very substantial one-way fee, The car has to be returned to its home country.
Almost all rental companies will not allow their cars to be taken onto ferries, so that pretty much rules out taking one from Italy to Greece.
We picked up a car in Italy and dropped it off in France. Yes there was a drop off fee but it was less than having to go back and return the car in Italy. As to dropping it off in Greece, it probably would be cheaper to fly.
Also evaluate the cost based on your most valuable resource that is non renewable: time.