As promised in another thread, I'm posting my suggestions for cheap yet magnificent Greek islands.
My no1 is Astypalaia. IMO, the most beautiful Greek island, with dead cheap accommodation and even cheaper food and booze.
The second one will be Kimolos and the third one Skyros. Both have great beaches, moderate to cheap accomodation but very cheap fish and alcohol.
An island that most Greeks know it for its Panayia but has amazing beaches, tons of things to see and can be done very cheaply is Tinos.
The island with probably the cheapest rooms all over Greece is Kalymnos.
Close to Athens, a nightmare in August, but very cheap and great for relaxation in Septemeber is Aegina.
Limni in Evia is a magnificent village, with cheap rooms, good and cheap food and most of times you will swim and see the dolphins playing!
Ithaca might not be considered cheap but if you do a good search for accommodation and follow GB's advice on where to eat, can also be done very cheaply.
Last but not least, Antiparos. It doesn't need a car, which means that you won't spend for petrol or rent, it has nice beaches, a beautiful -yet tiny- chora and if you are into some adventure, you can explore the caves.
Please add your own!!!