As an undergrad forced on a board plan, I easily eat over $20/day. I can't imagine that grad students are expected to eat at less than $10/day. I remember being at Stanford 4 summers ago and the lunches were pretty expensive on campus. A burrito or burger alone would go for $6, so I would guess it's up to $7 today. So basically grad students can't afford to eat at the cafeterias?!
I plan on eating out for most of my meals. I'm not a fan of cooking and sandwiches, while cheap and convenient, get old fast. I think quasi-fast food (eg, Chipotle or Baja Fresh) would go for $6-7/meal.
For COL purposes, I'm in SoCal (ie, about the same level as San Fran Bay Area).
BTW, to the poster above who mentioned $22.5/year for a stipend, that sounds fairly low for the area today. I think that was a standard stipend a few years ago.