Phil Miller reports that the union difference in wages for faculty was about 3.9%, and there is substantial difference between the fields. In general, those in the humanities received higher union differentials, and those in the professional or business fields who taught at unionized faculties received lower wages. As Phil notes, it's unclear where the report puts economics, though it looks like it got lumped in with other social sciences. I recall my colleague Mike White did a paper in Economic Inquiry about twenty years ago on the NFLPA and found that its existence helped linemen more than it helped those who play the "skill positions" (I never have liked that phrase -- good blocking is a highly honed skill.)