My taxi driver friend was telling me recently that the two terminals at Dalaman charge different rates for the same journey. The logic being that Turks are the major users of the old, now Domestic, terminal and they will not pay extortionate rates. The International terminal, being used by mainly foreign passengers, is a happy hunting ground for higher chargers.
Yes, it is negotiable. On a recent trip with more luggage than the vehicle would carry, a taxi was used to carry the excess at 25tl less than the standard rate to Fethiye. The rate was, it goes without saying, negotiated by a Turk who just dismissed the first price quoted and haggled a reasonable rate.
Ian