<p>I found the stipend at UChicago to be above average among the schools I was accepted by (and cost of living in Hyde Park by far the lowest), and am finding it much more than enough to live off–I’ll be surprised if I spend half of the $6250 estimate your link lists this summer. Except for students with dependents I haven’t really heard any complaints about the stipend here.
As for alternate sources of income, during the summer a lot of students in my department volunteer for the REU or teach summer school (there’s probably less of a demand for that sort of thing in math than Egyptology…)</p>