I'm with Travs on this one. With my expertise, I could have gone into the oil industry, civil engineering, etc and earned a lot more, but would have had less security in case of a downturn in such a sector than I do in academia (although my security in academia has gone out the window, but that's another story).
I remember a professor of Maths at Sheffield University (who worked in areas of maths with industrial applications; and note: "professor" is top grade within a department) telling me that his 31-year-old daughter was earning more than he did, and his younger daughter soon would be. I think they worked in finance or insurance or something similar.
I was recently contacted by one of my former students, now working as an actuary; a 2015 graduate, but I suspect he may already be earning more than I am. I'm not complaining: I have had a satisfying career, I have certainly earned enough to live on and, just to upset Oracle, I will have a Defined Benefit pension.
At the lower end of the pay range, I'm sure WP's observations are true; I think the difference between the top and the bottom is much greater in the private than the public sector. How much more would the £100k public sector manager be earning in the private sector?