What are the 25th-75th percentile ranges for the annual income for graduate students in the United States?
Note that those statistics are often averaged over all possible disciplines, and therefore, since there is such a wide disparity of stipend levels between different schools, and between different disciplines at the same school. 

At the institution I attended for graduate school, engineers had stipends approximately 30% higher than the science majors (chemistry, physics, math, etc.). Similarly, at the undergraduate institution I attended, a similar disparity existed between science and humanities graduate students. 

It is also important to ask if master's students, who often don't receive a salary at all, are included in that average. (And, since humanities students tend to stay longer, they may skew the statistics even more than one might expect.)

(One final note to directly address the question: competitive national fellowships in the US currently pay between $30,000 and $35,000 per year as a stipend. I would estimate, then, that most stipends are significantly below that amount. I'd say something in the range of $20,000-$30,000 would be appropriate in STEM fields, and probably $15,000-$20,000 for full-time humanities PhD's.)