However the source is quoted as being from 2009, so you need to add a bit on for the intervening 3 years. Allow €35 to €40 by day, or €45 in the early hours of the morning.
Alternatively take the direct bus (X96) which runs every half hour, for €3.50.
You can get the metro from the Airport. To the centre of Athens it costs 8 Euros one way for one person, 14 Euros one way for a 2 person ticket. You would have to change lines at Monastiraki station.
In 2010 I paid 45 Euro. That was during the day . Now would guess at 50-55 Euro during the day. The bus is a good idea if you can do it , reliable, nice and new and cheap.