In psychology/cognitive science programs I found huge variability. At state schools like Illinois/Michigan, funding was as low as $16,000/year (not including summer), and even in ridiculously expensive La Jolla, UCSD was only offering $18k. At prestigious private schools (like WashU and Northwestern), it was $19/20k. However, at a few private institutions with special IGERT grants, I got offers of $30k (for 2-3 years, then ~$25k for the rest), and at Stanford, it was $31k, plus a kick of $5000/yr if you got outside funding. So from my experience (at least in the social sciences), funding is more dependent upon the type of school (public/private) and also where the funding comes from (i.e., the particular lab, the department, the university, etc.).