I have a PhD. I really think it only makes sense if someone else is paying for it, your school or possibly your employer. It will make you very little extra money, but it for sure makes you stand out. You're no longer "candidate X", you're that guy with the PhD, which gets you noticed. (I've had headhunters tell me exactly this.)
If you begin a PhD, you have to understand a few things (in my opinion, please discard this if you don't wish the unsolicited advice.)
-You better have something to say. It's ok to undertake work that no one understands, but you better be able to explain at last a piece of it in about three sentences. I've had colleagues who when asked about their work have told the person asking they literally weren't smart enough to understand the answer. My colleagues were correct by the way, but it comes off as being a jerk. So... what do you want to study? It probably won't be what you finish with, but you must start with something or you'll flail about for years.
-Get through the program. Three years part time is a fantasy. The academic work is easy if you have a masters, it's the dissertation that kills people. Comps are, in my opinion, no big deal if you're working with a committee already. But never forget you're not there to learn anything, you're there to get through the program. You can't help but learn something along the way, but that's not the objective. A fair number of people keep working past where they need to in order to add to the science- do that in post grad. Ask your questions, answer them, graduate.
-It's isolating. Lonely. Literally no one on the planet understands the work you're doing and therefore cannot sympathize with you.
-It's selfish. Everyone around you that you love pays the price for your degree because of the time and energy you'll take from them.
-No one takes a new PhD seriously. In five years people will consider you a "real" PhD. Until then...
-I've heard it say that you have to learn to "think like a lawyer" in law school. PhD work fundamentally changes your thought process. To this day I have a hard time reading long form fiction and can point directly to my dissertation years as the reason. This isn't necessarily a bad thing, but you should be aware of the change.
-The insecurities of people you meet will manifest in ways you can't even imagine when they find out you're a PhD. Suddenly everyone you meet has a brother who got into MIT or they were accepted into a rocket science program but decided on art school instead. It can be uncomfortable, sad, funny, or just odd. And you'll have to live with it. (You ever see the youtube videos of a guy getting gas in his Ferrari? Everyone comes up to talk about how they used to have a sweet Mustang/whatever. It's similar.)
-Your Committee will ride you hard, and the day you tell them "No, I'm the expert. You guys are all wrong." Is the day you become a doctor. You might not notice it.
-Most people don't finish. I would have quit if I didn't have to pay my employer back.
-If you decide to proceed, find big datasets you didn't have to collect and use those for you research. It'll take two or three years off.
-As a PhD your opinion is worth more even when it shouldn't be. You'll protest that you don't know anything about zoology, but people will think you're just humble even though your PhD is in business (and yes, even a business PhD is a scientist. You might not wear a lab coat, but it's the same work.)
-You'll be expected to teach, even if it's a little. You can quit after a while, but it's a part of PhD credibility. It's easy to do it part time for a bit and then say you just didn't like it.
-You can decide how much you want to trade on the degree. I only have people I really like, or really don't like call me doctor. As an aside, the people you like (friends and family) will brag on you more than you know. "Oh, you're Sally's PhD cousin! I've heard about you!"
-That's right, you become the title. Similar to the military, where you're "The Captain, The First Sergeant, Whatever," you become "Doctor." Even if it isn't to your face.
-I discourage DBAs. Nobody knows what they are and you might have to explain it. At that point you're behind. It's a real shame, too. Every DBA I know is a genius and has done very solid work.
-If you quit, well, that's another discussion. It's tough to unring that bell.
Good luck on your decision!