I'm a prospective psychology PhD student, and one of the most intimidating problems was tuition expenses, but I've checked a number of PhD programs (NYU, Carniege-Mellon, Princeton) and they all say something along the lines of "anyone accepted into this program has fully funded tuition" and it feels too good to be true. Is it? Are there any expenses as a PhD student or am I studying for free?
The first thing to remember is this: "Free is too expensive." Having said that, most universities provide between 2 and 4 years of "soft" (i.e., university or advisor TA and/or RA) funding. The problem with such funding is twofold: (a) too many mouths to feed and (b) too little consistency in the system. You should consider writing your own grant before starting your PhD program or at least within the first 18 mos of starting your program. This will do a number of things for you, including; (a) focusing your research, (b) building a reputation, and (c) reducing your reliance on others.
In the US, most doctoral students are funded, usually with a Teaching Assistantship. Almost all TA positions come with a stipend and waiver of nearly all fees, including tuition.
There might be some exceptions. And there might be some fees, but, compared to tuition, they are low.
In some fields Research Assistantships might also be available, probably grant funded, and also very likely to include tuition waiver.
Of course, this limits the number of students to the funding available. But TAs provide a valuable service to the university in assisting in undergraduate education.