How much does collective bargaining matter? On Twitter, Will Wilkinson asks for data. I find this web site specifying the average Virginia state employee to be earning $50,298. Rortybomb says that for Wisconsin the comparable number is $48,267.
Yet Wisconsin had collective bargaining for state employees and Virginia does not. Of course this comparison is a gross one and it is not holding constant the composition of each work force, seniority, cost of living differences, and it also does not seem to pick up possible differences in benefits. Furthermore it does not consider the 48 other states. Yet, crude as this one-to-one comparison may be, it is more empirically sophisticated than most (all?) other discussions I have seen.
This David Blanchflower and Alex Bryson paper (see pp.9-10), using 1980s data, finds a union wage premium, for state employees, of 14.5 percent, with the premium being strongest for unskilled workers, as is the case in the private sector as well. (NB: I am not sure if they are adjusting for differential benefits but I think not.) Alan Krueger tells us that the union/non-union wage gap is smaller in the public sector than in the private sector ("overwhelming evidence").
I'm not pushing any particular answer, I'd just like to put the question on the table. What else do you all know?
Addendum: from Adam Ozimek: "The regression coefficients on page 8 of the report show that the union wage premium is between 15% to 16%, while the public sector wage discount is around 11%, meaning unionized public sector employees are paid 4% to 5% wage premium." Adam also provides further references and discussion.