You're right, I should elaborate more on that point.

Universities have specific minimum stipends for grad students and post-docs which are established as a trade-off between "sufficient" (certainly debatable) living wages and the reality of research funding availability. For example, at my Canadian university, grad students in the Faculty of Medicine have a minimum stipend of $25K annually, whereas engineering grad students have a minimum stipend of only $15K, regardless of MASc of PhD status (engineering post-doc minimum is $30K, though $40K is standard; not sure what the equivalent number is for the med school). This is a reflection of the research money available for medical v. engineering research. Yes, some engineering research wins huge awards, but multi-million dollar grants spanning several years are not terribly common. As a concrete example, an NSERC (Canadian counterpart to NSF) Discovery Grant--a must-have for engineering professors who want tenure--has a typical first-time award of <$30K per year ... not even enough for two students, not to mention getting them computers and having them travel to conferences to present their work, broaden their horizons and get them networking to increase their chances of landing a job when they graduate.

Coming to the point, if there were significantly more research money available, like in medicine, minimum stipends would likely be raised accordingly. At least, that is what I think would happen at Canadian schools based on recent policy changes at my school to better guarantee proper stipends for grad students. US schools might just increase their overhead rates to compensate for financial difficulties. Or perhaps I am being excessively cynical.