In my opinion, I would not ever do a PhD without funding. You have a degree at this point, so you should not ever have to pay to go to school (especially if you are helping the school by doing research there). Then again, if PhD is really your dream, and you don't have problems taking out loans, then go for it.
All I know is PhD in engineering, and I don't think the degree helps you make that much more money (if anything, it might hinder some jobs since you may be too specialized). I cannot speak for other majors, so I'm not really sure of your background! But anyway, PhD is an expensive endeavor for 5 years, so you should not be taking out the loans.