I just pulled a few 990s for professional engineering societies. Except for their CEO/ED and one or two key regional managers, they make far less than $400k/year. That said, I looked at PLOS's 990 and saw numbers that were a) much smaller than I expected based on the $400k comment and b) in line with but slightly higher than what I saw from, say, the Society or Petroleum Engineers or Society of Automotive Engineers.

Make sure you restrict to major cities. That affects comp. Also I suspect those forms show base pay not total comp which is typically far higher.

Not sure about the legal reporting requirements, but since my wife works at one of them I can tell you first-hand that they don't get much on top of their base salary. Non-profits are 80%+ salary compensation, even at the highest levels.

Also, engineering societies generally are based in major cities.