Don't forget to add in the fact that you could have been earning that non-zero salary potential for two years instead of paying tuition...

When I started my Ph.D., early on they ran the numbers with us. The increased starting salary from having a Ph.D. (probably somewhere in the $15...$20k range... I forget the exact number) will never make up for the four year head-start someone who stops at a Bachelor's degree gets on actually drawing a salary, once you discount to net present value (even when you factor in a graduate student's meager stipend).