The American Association of University Professors recently released a report on the financial situation of professors. One interesting datum apparently gleaned from the report is a ranking of universities by full professor salaries.
I have heard it said that Harvard pays below market because it pays in prestige, but that doesn’t jive with its industry-leading $192,600/year (keep in mind this is average for full professor, which is rarely achieved before one’s 40s at best).
One interesting fact noted shown in figure 2 of the report itself is that while, yes, PhDs do earn less than professional degrees (law, business, medicine, etc.), the difference is, on average, not nearly so large as one might expect. In 2007, the average PhD made around $95,000, while the average professional school graduate earned about $115,000 (both numbers are down, incidentally, from 1997).
That said, the ceilings are probably different. The average full professor at Harvard — arguably the pinnacle of the profession for someone with a PhD — as already said makes just under $200,000/year…or about the same as the typical big-firm lawyer a couple years out of law school (though perhaps not this year).