The taxes really depend on the airport involves during the flight. On a recent return flight from Dusseldorf->London Heathrow->Dubai->Melbourne, on FF points, the tax was 1/3 cost of the full price tickets, (around 450EUR).

Airport taxes in US must be quite low, relatively.

It's that way in Canada. Both Aeroplan and AirMiles make you pay taxes and fees. Depending on which airports you cross that's probably $50-$75 a head each way.

This has been my experience in Europe with Star Alliance. By the time you pay all the fees you're paying the same as in a low cost airline.