Fully-funded econ PhD candidate here. I got paid a small stipend and had tuition covered in exchange for working as a teaching assistant while I was "going to school," i.e. taking classes. Now while finishing my dissertation I'm teaching two classes and getting paid the same as an adjunct prof, plus working 10 hours a week as a research assistant at $20/hour. Making enough to get by, but would be tight if my wife didn't have a decent job.
Seems like a pretty good deal for the school - graduate instructors teach the classes full profs don't want to teach (including online classes which are a big source of revenue for the department), and generally score on par with full profs on course evaluations. Pretty straightforward exchange - we get an education, a degree, and ~30k per year; the university gets our labor. It's basically a teaching/research job where part of the pay is a graduate education.