Okey, when you calculate your salary why you are missing the stipend and tuition waiver that you get as a package while going for PhD. You will get about 25000-30000 in stipend and tuition waiver which makes your direct and indirect income above 45000/yr for 5 years while doing PhD. You cannot ignore this fact while calculating how much money you will make with or without PhD. Now, if you do the math, you make a lot more with PhD degree, and not to mention stability and if you get into academia, a tenured track position, that 70-90 K is a 9 months salary. You can bring your own money for research and get paid by your grant money in summer plus you can take a break. There is nothing like 8-5 in academia, you will have much more flexibility than regular job. Ofcourse you will spend 80 hr/week getting less than minimum wages while doing your PhD but it is worth it if you think you can finish it and if you have a passion for the research that you are developing.  It is a commitment of time and it takes a lot of patience to finish it. 30 percent of PhD students don't finish it even after 7 years.