I'm single, rarely eat out and my groceries are running about $500/month. Most of my groceries come from Costco. That amount excludes alcohol and includes very few cleaning products (I have a cleaning lady) but does include supplements, which can get pricey. My diet is mostly vegetables, yogurt and lean proteins.
So, yeah, that diet is expensive. Yesterday I was near a higher-priced local chain and wanted to get tofu, which I haven't been able to find lately at Costco or my regular grocery store. It was $6.99 (plus tax) for a 10.8-ounce carton, which works out to about $10/lb. with tax. I just checked the on-line fliers at my usual grocery store and I can get a boneless quarter ham, lunch meat and hot dogs for less per pound. Of course most come with more fat, more sodium and, in the case of lunch meat preservatives. Ramen is probably still cheap but it's a fat and calorie bomb. I buy Great Grains bread- wonderfully loaded with sprouts, seeds and texture but $8 to $9 per loaf.
If I were determined to eat as well as I do now on a budget I'd have to shop a lot harder for specials and probably buy more pork and chicken and less seafood (shrimp is my only real splurge in that category) but I bet I still couldn't get it below $400 unless I started cutting out protein powder and supplements.