I love Costco too, but our apartment is too small for storing lots of bulk food items. My wife and I dream of having a bigger kitchen, pantry, and chest freezer someday.
I'm working on getting better numbers, since it seems like a lot too, especially when I think about what I'm eating: toast for breakfast, leftovers for lunch, simple dinners. I'd really like to know!
When I was single I was averaging about 250 -> 350 a month. But I offset this by buying chicken in bulk at 40LB increments and that usually lasted me several months.
For two right now I'm average about 700 to 800 a month. I have a number of food allergies so ingredients or restrictive. The other half is as close to organic as possible.
I'd say the biggest contribution to the price is moving off farm sourcing. I don't have the time right now to process 40lb of chicken, or several cases of produce.
Also that it seems like nobody has more than about three days of food in their pantry. My mother still puts up hundreds of quarts of canned vegetables every summer out of the garden, freezes enough other stuff to fill an entire full-size freezer, then they buy half a beef-creature from the farmer down the hill to fill the other freezer. And usually a couple hundred pounds of potatoes. There's probably enough food to keep them alive for two years.
Driving to pick up dinner each day would be just tedious.