Car rental costs in Greece / public transportation

Both your questions depend on the location you are visiting. For example transportation in big cities like Athens and/or Thessaloniki are sufficient, well known destinations as well like Mykonos or Santorini but other places don't have often schedules or are missing interesting routes. In my opinion renting a car is always the best and cheapest way to visit places in Greece. Prices vary from place to place and depend on the season. For example the highest prices are during July and August but before and after that they drop significantly.
I would suggest 3 different car renting companies since i don't know which place you are visiting.
For a car rental in Athens visit bluecar.rent their prices start from 15€ / day if you book a month or so online.
For a car rental in Ios Greece i would suggest trohokinisi.com they have good prices and can deliver the car anywhere.
For a car rental agency in Milos i would suggest milosrentcar.gr as they are the cheapest car rental agency in the island and they have vintage cars as well. I have used them myself a couple of times.
So, for example in Athens the taxi fare just from the airport to your hotel is between 35€-55€ which is a 4 day car rental worth. The only downside is that YOU have to drive
I hope this helps.