That's not entirely true at the places I've been and people I've talked to. The reason is because a lot of that overhead gets waived if the professor teaches.

That's not entirely true at the places I've been and people I've talked to. The reason is because a lot of that overhead gets waived if the professor teaches. Usually one class/year = 1 grad student. That's why many grad students (PhD candidates) TA at first too, because unless they are really, really good, they aren't going to be worth it to give them an RA position, especially if they are taking classes and they can't be productive enough for a professor. In a large group, usually a few professors will teach, and the rest will supplement grad students with NSF or some other sort of grants. The last NSF grant I helped with, done by a professor I worked for, was essentially for the grad student's salary they get to live off of, as the rest of the costs were waived by the school (in this case, 25k/year for 3 years)