Might be best to avoid Greece at the moment. The currency is going belly up and the whole country could go bankrupt. Why not try one of the comparison sites like LowCostHolidays.com. But be careful of the additions for luggage and airport taxes if you go to Spain. Good luck!
When you say cheap,we assume you mean cheap cost of going out, buying food, drinking,eating out etc when there. Nowhere in Spain/Greece is cheap as it used to be anymore. Last year i was paying around 6- 6.50 euros for a pint of local and a short and mixer as an example. Food in supermarkets and eating out has also gone up.
On cheap to get there,I think you may find some good deals
Just been to Kos fully inclusive for a week. Very cheap and the service was excellent. Ramira Mitsi Beach hotel near Kos town. Try it out via the web and see. Fantastic holiday. Good luck.
If you can manage last minute deals that's the best way to get cheap holidays.
A few years back I managed to get a holiday in Kos, B&B, 4*, for £115 pp. We had two days notice...which was fine for us. Got that through the travel agents who do all the searching for you :-)