Many graduate students don't have a stipend. So, the poverty line in the US is ~12,000$ per year. But, I have know graduate students to live comfortably on as little as 9,000$. Granted, they often had a lot of social capital and some family connections. However, just knowing they had a ticket out often made living in a van or whatever acceptable.

But, I can also point out a friend who graduated from collage in 2002 without debt working at Denny's for 9$ an hour at who also supported his wife and 2 children. (He did get a 13$ / hour internship his senior year, but it was only 20 hours a week so he kept the Denny's job.) What's important is he knew it was no sustainable, but he saw a light at the end of a tunnel that made 80-90 hour weeks acceptable in the short term.

PS: While poor is often transitory, poor and uneducated, hopeless, addicted, and desperate is rarely transitory.

I've never heard of a $40,000/year grad-student stipend. The median stipend, AFAIK, is $20k/year for about 50 hours/week of work (when you actually factor in teaching, research, everything beyond the "student's" curricular responsibilities).