PhD programs are free
. You read that right. Students in PhD programs are funded by a combination of the university and their supervising professor who gets grants and in turn pays his grad students.
At the better colleges more of the money is given in terms of scholarships/grants and less in terms of work-study. For work study, thats what the TAs (teaching assistants) and RA's (research assistants) due to work their way thru school. Bottom line, though, is in PhD programs you are actually paid
to go to school! Granted you're not living high on the hog, but enough to survive and medical coverage is usually included.
See, for example, http://www.yale.edu/graduateschool/f.../standard.html
for an overview of financial aid for grad students.
If you were accepted into the Yale Sociology PhD program, for example, a web page says "All accepted students are fully funded for five years. In the first four years they receive full tuition and a living stipend and in the fifth or sixth year students are eligible for the dissertation write-up fellowship. In the third and fourth years students are required to serve as teaching fellows to receive their stipend. In 2003-2004 the annual stipend was $16,000.
In other words, not only can you spend up to 6 years at Yale tuition-free, they will kick in an extra $16K/year for you to spend as you wish (a lot on housing, no doubt) and all they ask is you serve as a TA for 2 of those years! See http://www.yale.edu/socdept/gradprogram.html