Fair point, but I would argue opportunity cost. It seems to me there is a differential for a year of experience vs a year of education. A simple example is the CHP; a masters degree counts as one year of experience, despite taking two years to complete typically, and the situation is far worse for a PhD (5 to 6 years to complete and only counts as 2 years experience).
If I spend 5 to 6 years at a job that has no real opportunities for career advancement simply so I can go to school for free for a degree that I'm not convinced has enough additional value to justify the time expense; I'm NOT spending 5 to 6 years in a job that is really advancing my career by providing me with the challenges and experiences necessary to really grow as a professional.
Right now, I really only see one clear advantage to having a PhD and that is having a career option for retirement; lecturing at a university. The other advantage that I hope exists, but am unsure about, is whether that PhD will make finding an ideal job easier or harder than with only a masters.
If we look at the salary survey for CHPs. . . all CHPs have a mean/median close to about 116,000/year.
CHP with Masters: 111,000 to 121,000 for the different categories.
CHP with PhD: 118,000 to 122,000 for the different categories.
So at best, about a 7k possible salary difference vs only a masters. Lets say that remains over the course of a career. Five years for a PhD fulltime study = five years without earning 111,000 dollars or a loss of half a million in income, to earn an extra 7000 for 35 years (assuming a 40 year career) = 245,000 recovered of the 500,000 lost (admittedly, I've failed to account for the time value of money over a 40 year period, but in general losing 500k today is far worse).