From the Chronicle of Higher Ed ($$$):
Inequalities also exist among professors at the same campuses, the report notes. Although the gaps in salaries among disciplines have narrowed, the AAUP points out that substantial differences endure. Assistant professors in the best-paid disciplines are still paid significantly more on average than those in English departments. For instance, in business departments, assistant professors make 102 percent more. In computer-science departments, they make 60 percent more.
English professors do not have the outside opportunities that business professors have, meaning the overall market values them more. Why should they be paid the same rates?
Update: here's another excerpt from the article.
The same logic seems to govern the hiring of football coaches at Division I-A institutions, who make an average of 9.4 times more than their full-time faculty counterparts, according to the report. While both have reached the tops of their games, there is a perception that football coaches generate more money for the university. That perception, however, may be a myth, says James L. Shulman, co-author with William G. Bowen of the 2001 book, The Game of Life: College Sports and Educational Values, which is cited in the report.
I have been a critic of coaching salaries (see here, here, and here), but not for this reason. My beef here is that salaries of head football coaches at D-1A institutions should not be compared to salaries of general faculty. A more accurate comparison would be to the salaries of department chairs with the assistant coaches being more comparable to general faculty.
Lastly, I haven't read the Shulman and Bowen book, but I wonder if they correctly frame the value of a coach.