When taking a taxi in Athens be alert. Ask for the price first or ask if he has a meter. In most cases it will should cost about 5 euro. (informally a flat rate for anywhere in the city center)
Be wary of getting cabs in tourist traps as they will likely rip you off.
We always asked this way:
"(location)? 5 euro?"
Because we knew our hotel (divani caravel) to city center was 5 euro.