For a family of 4 plus a cat, we spend around $500/month on total groceries and household items, so probably $300-400/month just on food alone. I don't track household items separate from grocery (since most comes from walmart), but that is a rough approximation looking at recent receipts.
We eat well at home (a wide variety of good stuff), since it is typically way cheaper to cook well at home than go to a restaurant, and you know what is going into your food health wise.
Like you, OP, we mostly shop at walmart and buy mostly store brands there (as long as the quality is not compromised which it rarely is). But we also shop at the local grocery store a couple times a month to take advantage of sales prices (if we can save enough to justify a trip). And we shop at Aldi's, a deep discount grocer. Lately I bought a bunch of high quality low price fresh fruits, veggies, plus dairy items there. The ethnic stores make an infrequent appearance, and provide higher quality ethnic ingredients than walmart, but at lower prices than walmart, plus fresh veggies and fruits at cheaper prices typically.
Fresh fish is about the only thing we have a hard time getting at these combos of stores, and usually settle for whatever the local supermarket has (which occasionally means paying full price around $8-10/lb - typically for sushi which is still way cheaper than eating sushi at a restaurant).
Right now we don't shop at Sams/Costco/BJs because we don't want large quantities of a lot of things (not a ton of pantry space, no deep freeze, smallish fridge, etc). The warehouse club may be an option as the kids get older and we need larger quantities of 1 item.
We buy a bunch of fresh fruits and vegetables weekly or more often. Some are more expensive than others, so we focus on buying the most quantity of the cheapest items (apples, oranges, bananas typically for fruit, and carrots, celery, green peppers, onions, frozen peas, frozen broccoli, zucchini, and squash). We don't have a real hard time getting these for $0.50 to $1 per pound, but will pay more if we really want them or for convenience (to save a trip to another store for example).
For meat, we usually buy when on sale and freeze for later. Luckily for our budget, we like cheap meat. I like london broil, top round roast, and the like (lean meats), chicken breast, pork loins, etc. These are typically available around $2-2.50 per pound. Ground beef or ground turkey is also around $2/lb when on sale or close to sell by date.
DW likes some of the fattier cuts of meat, which in her case means cheaper meat. Chicken thighs/legs are $0.40-$0.50 on sale, and pork shoulders are under $2 all the time.
If we are cooking fish, frozen fish is ok. Salmon, flounder, tilapia, etc are usually $2-4 lb on sale and taste just fine to us.
Bottom line is that most fruits and veggies are $1/lb or less, and meats are around $2/lb on average.
Edit: about to go throw some carrots, celery, squash, zucchini, onions, green peppers, green peas and fresh sliced up steak into a stir fry and serve with rice and lo mein (made from $0.18 ramen and $5 sesame oil imported from japan! - the good $hit
). Mmmm good eatin!
There is a wide assortment of organic fare, including (Alvarado Street organic bread). I am a slow cooking fan, so I do get bulk oatmeal, nuts, etc. at the co-op. It would be nice, though to be able to find toilet paper, tissues, toothpaste, foil, (mainstays) for a lower price. 
