This is a good exercise for me to go through, too, so I'll try to figure it out for myself.
For groceries, I'd probably go through this in a week, roughly:
A box of cereal - $4
Half a gallon of milk - $2
Half a pound of coffee - $5
A loaf of bread - $3
A pound of cheese - $4
A pound of deli meat - $6
A couple of pounds of fresh fruit - $3
A pound of ground beef - $3
A pound of chicken breasts - $5
Various vegetables - $10
Snacks - $10
Throw in $10 a week for the couple of staples I always seem to need -- flour, peanut butter, etc.
So my weekly grocery budget comes to $65, which is $260 a month. Then say you spend $2 a day on coffee or pop every day that you're at school: another $10 a week, which comes to $300 a month altogether. Grab a sandwich twice a week, and that's another $10 -- $340. Go out for brunch or dinner once and that's probably another $20 -- now we're at $420 a month, or about $13 a day.
That kind of sounds like a lot, but I think it's fairly realistic for me.