Three suggestions, in random order, with decent prices and good transportation to/from the airport, the NTUA campus, and the center of the city.

• President Hotel Athens

  Single room: 60€. Double room: 60€. Triple room: 82€. Prices include breakfast and taxes.
  Meal supplement: open daily menu from 17€ per meal per person.
  Important: You are requested to book by filling the provided form and sending it by e-mail to the hotel no later than July 9.

• Best Western Ilisia Hotel

  Single room: 60€. Double room: 70€. Prices include breakfast and taxes.
  Important: Book by e-mail (not from their web site) and don't forget to mention NTUA/PLS9 to get these prices.

• The Golden Age of Athens

  Single room: 60€. Double room: 65€. Prices include breakfast and taxes.
  Important: Don't forget to mention NTUA/PLS9 in your reservation, to get these prices.

Of course, Athens is full of hotels and, if you're looking for something different or a different price range, you will be able to select where to stay from a very wide list.