There are 3 taxi companies operating there, all charging USD15 (incl tolls) to the Old Quarter. The taxi ranks are right outside the arrival hall.

You can also take a van which charges USD2 per person but they only depart when the van is full (and when I say "full" I mean every square inch of space has been taken up). They will drop you off somewhere near the Cathedral and you will need to deal with the taxi mafia to get to your hotel from there.