A taxi should cost you €40 to €50 from the airport to the city centre. There’s supposed to be a flat rate, but it doesn’t always work like that!It's a long walk (20 mins) from the passport control to the metro.
Yes, cabs are plentiful, but not cheap - it's not a ten minute journey into central Athens and the metro is pretty good - I was last there in July and I think it was €9.00 to take the metro to Monastiraki or Syntagma (Blue line)