Via the Chronicle of Higher Education (subscription access only), covering 4-year institutions for the academic year 2005-06.  The average full professor of philosophy (note:  philosophy is lumped with religious studies) earned $82,030, compared to $76,413 for an English professor, $80,706 in History, and $84,059 in Mathematics.  The average new Assistant Professor of Philosophy earned $46,780, compared to $45,882 in English, $45,723 in History, and $50,151 in Mathematics.  Average salaries at the major research universities are, of course, higher (for example, the average salary of a full professor, across all fields, at a doctoral institution is over $100,000), but this data on averages across all four-year institutions probably gives a more realistic picture of compensation.