The point remains correct, however: the foregone salary is about 300,000, plus tuition if one is unable to get a waiver. The differential is 15,500* 30, about 500,000, very comparable.
To make the computation correct, one has to discount the stream of cash flows differentials, whereas the foregone salary occurs up front. The net present value of the salary differential seems about the size of the foregone salary even when tuition is not taken into account.
To summarize: (i) one cannot judge whether it is wort it to get a Ph.D. by looking at the salary, and (ii) the monetary value of the Ph.D. is negligible.