A large determinant in how much faculty earn is the discipline in which they teach, according to a report at Inside Higher Ed on the salaries of tenured and tenure-track faculty at four-year colleges and universities. The best-paying fields for full professors are law, business, and engineering, with humanities fields towards the low end. The article notes:
The large salary gaps for new professors may understate the financial impact of these differences. Generally, those in the higher-paying disciplines are also in fields where time to degree for Ph.D.s is considerably shorter than those in the low-paying fields. So those being paid the least have taken the longest to be able to apply for full-time jobs, and on average have more debt.
The article also reports that there has been a 2.2% percent median salary increase over the past academic year.
Philosophy data is lumped in with religious studies:
You can view the data for other disciplines here.