How much a university or college professor makes depends upon her chosen field. According to an article in the Chronicle of Higher Education, law professors earn the highest salary (an average of $136,634) and Library Science professors earn the least at $77,583.(see Reference 1)
A Ph.D. in a medical field doesn't equate to a large salary. A physical therapist (PT) with a Ph.D. could expect to earn just $59,625 according to Payscale.com. A nurse with a Ph.D. would not earn a higher salary than an RN (unless he chose to enter an academic profession).
Research scientists earn less on average than university professors as a whole. Payscale.com reports that while the average professor in 2009 earns $90,147, a research scientist working in biotechnology made $78,221, and scientists in other fields could expect to earn $74,768. (see Reference 3)
When it comes to salary, students are no better off graduating from a prestigious private university than a state university. Students who graduated from MIT with a Ph.D. and reported their annual salaries in a school survey, earned anywhere from $30 (not a typo) to $164,000--about on par with other graduates countrywide. (see Reference 2)
A Ph.D. is a significant investment in time and money which rarely leads to a lucrative career. If you're looking to get a Ph.D. to earn more money you're looking in the wrong place. If research is your first love, by all means pursue a doctoral degree.