Location, travel time and level of service are the main criteria that determine the cost of accommodation in Athens. Given the star rating of your chosen hotel, prepare to pay 50, 70, 100, 200 EUR per night in a standard room (in 2*, 3*, 4*, 5* hotels, respectively).
To pay for accommodation in Athens inexpensively, use the services of hostels - even in the city center you can find establishments offering overnight accommodation for 12 EUR per person. Hostels usually have rooms for 1-6 people.