Is it worth it? From a financial standpoint absolutely not. In fact, I would go so far as to say the ONLY benefit to doing it would be the personal satisfaction you'd get out of it. If you think that is worth your while, go for it. I would like to say that education doesn't have to take place at a university and come with credentials though. If you're smart and motivated, you can just as easily read tons of books on your own, read various papers, etc. Producing any content yourself might be a bit harder, but is that something you are really interested in? Also, understand that a PhD is very little like a BA. In undergrad you take a bunch of classes. That's pretty much it. For a PhD you will take some classes, but the emphasis is on work. Your PhD will be based on your dissertation, which in history could be something as specific and mundane as citation methods of 17th century England. That kind of stuff is fairly common... it is more than likely you will spend 5+ years of your life working on something that no one else in the world, or perhaps only a handful of people, actually care about. At the end you will get to say you have a PhD though, which is a big motivator for a lot of people (certainly not for me though). It reminds me of a scene in "
Searching for Bobby Fischer" where Ben Kingsley's character tells the kid if he works really hard he'll get a special certificate to motivated him, when in reality the special certificate was just some random printout with no value.
Of course the above is my opinion. People who pursued PhDs will likely have different ones.