It depends on the schools. A graduate student at a UC school is way cheaper than a graduate student at Stanford or Caltech. It also depends on the PhD program,

It depends on the schools. A graduate student at a UC school is way cheaper than a graduate student at Stanford or Caltech. It also depends on the PhD program, or more to the point, on the course requirements. If a PhD student already has a MSc when he enters the program, he can start doing research right away, and his advisor get a return on his grant money way faster.