Aside from the cost to the professors and other students - which others have already outlined - there is also a personal cost to you.
Master’s programs are designed for, well, master’s students - students who are going to be there for 2 years max and want non-academic jobs. That means that all the classes, co-curricular and extra-curricular opportunities, networking, and career advising are designed for that cohort of students. You’re going to get advice, opportunities, and coursework that’s specifically tailored towards graduating someone with a master’s and (presumably) a professional career.
PhD programs are designed for students who are in it for the long haul. The expectations, opportunities, career advising (or lack thereof), etc., are all aimed at a student who is expected to spend at least 5 years in the department and who is aiming for a research career.
What does this mean in practice for you?
-In a master’s program, your program advisors are interested in doing career advising from day one because it makes the school look better if you have a job secured at graduation or quickly thereafter. In a PhD program, career advising doesn’t really begin at day one unless you have a really unusual advisor, and in fact, many advisors think it’s better to hold off on really tackling career discussions with their doctoral students until about year 3 or so. (The professors in this forum are excellent about being more proactive, but also - in my experience - unusual.) And in fact, sometimes asking about career stuff too early and too repeatedly, especially non-academic career stuff, can signal to a professor that you are “not serious” and thus not worth their time and investment.
-If research is not your passion and you are not interested in a research career, prepare to be disappointed and perhaps miserable. Doctoral students are expected to do research. You’ll likely have to serve as a research assistant for at least “20” hours a week (but 20 hours is rarely actually 20 hours), and you’ll be expected to chase the kind of milestones that are important for someone preparing for a research career: publications, presentations at conferences, brown bags, etc. You are going to be expected to do that instead of, not in addition to, non-academic professional career preparation. And in fact, many professors will consider any time spent doing non-academic career prep instead of research and writing as a waste of your time (and a signal that you are, again, “not serious.”)
-Master’s students who want a job will do well to do a summer internship somewhere, and perhaps even an internship during the academic year. In fact, your program will try to help you FIND one. Doctoral students are expected to do research during their first summer (especially since summer funding often comes directly out of someone’s research budget). Nine chances out of 10 you won’t be able to do a professional internship anywhere; your program may even forbid it. And 9.9 chances out of 10 you won’t be able to work part-time or do an internship during the academic year. You won’t have time, and your program may even prohibit it formally.
-Sometimes some programs have different classes for master’s and doctoral students. Mine did. The doctoral versions are usually more theoretical and less applied than the master’s classes, and they may be less useful to you. The master’s versions often have practica or simulations attached that you will miss out on as a doctoral student. Trying to take the master’s versions will at best get you long, unwanted conversations with your advisor and at worst blow your whole plan pretty easily.
-If you’re going to a professional program, the non-terminal master’s you get is often different from the master’s that the terminal students earn. For example, I got my PhD in public health. The non-terminal students got an MPH in my field, but the non-terminal master’s we earned was an MA. You could, theoretically, earn an MPH on the way to your PhD, but it was a lot of extra work and planning (different classes; see above) and again would earn an unwanted “talk” between you and your advisor.
There are other things that are less direct but no less annoying:
-In a master’s program all your classmates are also trying to get professional jobs and get out in two years. In a doctoral program, all your cohort-mates are in it for the long haul. This will have an effect on your psyche in a way that’s difficult to explain. Let me just say as someone who completed a PhD program, peer pressure is real.
-It’s really hard to hide your intentions as a doctoral student. When I say hard I don’t mean difficult; I mean psychologically taxing. Doctoral cohorts tend to be small - maybe about 4-8 students on average - meaning in that your entire department, maybe there will be around 20-50 doctoral students. Your professors expect to see you around; they know who you are and they watching you; they are listening to you; they are scrutinizing you. Even if you think they don’t know who you are…they do, lol. Frankly, this is psychologically taxing even if you want the PhD but think you may want to go the non-academic route; I can’t imagine the feeling if you KNOW want to drop out as an MA. (I’m not exaggerating.)
-You’ll be expected to do some longer-term planning, so you’ll have to do a lot of unnecessary work - like prepping your dissertation work early or something.
And lastly, but importantly, sometimes getting that non-terminal MA or MS takes more time than you think. Many professors will (rightfully) assume that the MA is a non-essential stepping stone towards the PhD and frankly will not care when you earn it, precisely; if there are specific steps you have to take towards getting it, they may not care about them as much as you do. I’ll use myself as an example: it took me 3 years to earn my non-terminal MA on the way to my PhD. The reason it did is because in addition to completing coursework I had to write and turn in a paper, but my PI (and I) wanted the paper to become a publication. So rather than turning in something whipped together quickly I spent extra time perfecting it and turned it in super “late”. I did end up publishing it (in a very good journal, actually) and I personally did not care that it took me 3 years because I wanted the PhD. But if my goal was to get out and my PI kept sending me back for more edits, I might have gone insane.
For all of these reasons, my recommendation to students asking this question is usually to go ahead and pay for the master’s degree. Some things are just worth the money.