we're staying at the following for less than $100 night for 2 people with breakfast included and it didn't get bad reviews (it probably got some good ones since i looked long and hard but can't remember exactly what now)
HOTEL ECONOMY
5, Klisthenous st. Athens
Tel +30 2105220520
Fax +30 2105220640
htlecono@otenet.gri booked on Travellerspoint.com but you can find it on some of the other booking sites - just google it.