It really depends on several things:
1. What do you eat? (Are you paleo, vegetarian? Do you eat organic, local?)
2. Where do you live? This is a huge factor. I live in California - you'd think that it's cheaper because we grow the stuff here, but overhead is huge. You can buy strawberries grown in my county for cheaper in Georgia than you can here.
3. How much do you cook?
What I've found for trying to cook for less:
1. Step one, figure out what you buy and figure out how to get it cheaper. Check ALL stores (including Asian/ Mexican stores) and keep a price book. It helps to have a hard copy for several months. The goal here is to get everything for rock bottom prices. If you eat a pound of pasta once a week, then buy it when it goes on sale for $0.50/lb. If this happens every 4 months, buy 4 months worth (16-20 pounds).
2. Step two, figure out the cost per meal for your favorite meals. And put the cheaper meals more often in your rotation. Beans and rice are cheaper than steak.
3. Step three, add cheaper meals to your repertoire - this may very much involve eating in season (loss leaders), and checking out websites like The Prudent Homemaker, Cook for Good, and Budget Bytes
4. Step four, don't throw away any food.
5. Step five, figure out how to make some things that you buy - yogurt, bread, granola, applesauce, smoothies.
For me, it's snacks. The snacks kill me. I can tell that my grocery bill goes crazy with the snacks. I'm focusing more on bananas and popcorn and less on crackers and applesauce pouches.
For my family of 4, I would find it very hard to hit less than $500 a month in my location, unless I went vegetarian. As I'm trying to give up wheat and drastically reduce my carbohydrate intake for weight control reasons, that's not in the cards. I still try though.