The relative price of a professor
In 1905 "G.H.M.", an anonymous college professor, wrote a four-page article for the Atlantic Monthly in which he pleaded for more money for college professor salaries, and claimed to be vastly underpaid. The first thing to note is the relative level of professorial salaries back then: he claimed that the "average college professor’s salary"--the salary that he saw as clearly inadequate and unfairly low--"is about $2,000." Stan Lebergott's estimates in the Historical Statistics of the United States are that the average annual earnings of an employee in America in 1905 were $490 dollars if employed for the entire year--or $451 taking account of the hazards of unemployment. What G.H.M. says is the average college professor's salary is more than four times annual average earnings of the time.I think Brad somewhat overstates how much things have changed. The average salary for full professors at private, doctoral universities is now $136,689. (Source.) To compute total compensation, you should include the value of employer-provided health insurance, contributions to retirement accounts, and other fringes. You might then get to around $160,000. (I am guessing that the 1905 prof did not get these fringes; there was no income tax to avoid.) I don't know what modern data are comparable to the Lebergott data, but average hourly earnings are now about $17, suggesting annual earnings about $34,000 for a full-time worker, with compensation (including fringes) a bit higher than that. The relative price of professors still seems to be about four.
Today's professors don't make such large relative salaries (except in business, law, and medical schools). In order to match turn-of-the-century college professors in terms of income relative to the national average, a professor today would have to make an academic salary of roughly $250,000--a height far above any professorial average, and one attained only by academic celebrities.
Update: The comments raise a couple of fair points. First, with a more comprehensive group of professors, the average compensation would be lower, and the relative price today might be closer to three than four. Still, a professor need not be an "academic celebrity" to make four times as much as the average worker: Being a typical full professor at a private, doctoral university will do. Second, with the average American more educated today than a century ago, one would expect this relative price to have fallen, as it may have done to some extent.