Yeah, as the two previous posts from filipe and cconstantine point out, we're talking about
net contribution.
This is why I mentioned the MBA, Law, Medicine, and Undergrad degrees as something apart from Ph.D programs in science and/or engineering. Since most people experience college as an undergrad, or maybe as a professional student, they think of a degree program as a situation where you get an education in exchange for money. So scholarships and stipends are a privilege, maybe even a kind of gift.
PhD programs are completely different. Universities use grad students to provide labor that at a fraction of the market rate - maybe 10%-20%.
By the way, it doesn't bother me that a person would be willing to apprentice to a professor in order to get a PhD and experience. That's the student's decision. But it irritates me enormously when I see a visa or green card used to force the worker to accept employment at severely below market rates.
The first is choice, the second is government sponsored indentured servitude, an egregious violation of everything I believe is good about free markets, democracy, and at-will employment (this is no exaggeration, I really do think when an employer controls the employee's right to reside in the US and hopes for getting a green card, it severely undermines personal freedom in a way that has no place in a capitalist democracy).