Actually, things are a little more complicated here. It's correct that SXF (and BER) are in another country (Landkreis Dahme-Spreewald, LDS) and don't belong to

Actually, things are a little more complicated here. It's correct that SXF (and BER) are in another country (Landkreis Dahme-Spreewald, LDS) and don't belong to Berlin which would ordinarily imply that the SXF Airport is not part of the area where Taxis from Berlin have to drop you off and that Taxis from Berlin would not be allowed to pick up passengers at the SXF airport for the return trip to Berlin (and the other way round, Taxis from LDS could not pick up passengers in Berlin for the return trip). Since that is obviously an unacceptable situation the "Berlin" area was expanded to include the airport and that LDS-based taxis could pick up passengers in Berlin in return. It used to be that the fee was slightly different depending on where the taxi was based, but I haven't noticed any difference in the last years. The reason TXL is cheaper is mainly because it's closer, SXF is pretty far out and BER is even further out. There's an extra airport upcharge for TXL though.