Matt Might, a professor of Computer Science at the University of Utah, created a set of circular diagrams to explain the significance and impact of a doctoral degree. He uses them to explain to incoming doctoral students what they're pursuing. The first, a circle, represents the sum of all human knowledge. Click on the link to view the sequence.Link
There's still the opportunity cost of going to school for an additional six years instead of working, and on average people do not make that money back. If you're in it for the money, the PhD isn't worth it.
Depends where you get it from and what