The loss of food culture in the US is partly to blame. All of the loopy fad diets people are describing here are a symptom of the same larger problem that has caused the obesity epidemic. There is no shortage of intact culinary cultures in the world that include wonderful, healthy, inexpensive things that anybody can prepare with a little time and love. Accordingly, your best bet for good, cheap produce and healthy stuff in a big city is any immigrant/ethnic market. These usually depend on parallel supply chains that allow them to undercut and outperform mainstream supermarkets.
The loss of culinary traditions I mentioned also includes the age-old habit of creatively re-using leftovers to make new things. Cook potatoes one day, grate the leftovers and make potato pancakes the next. In so many ways the vast generational shifts in food culture in countries like the US over the last century or so are a direct reflection of a dozen or more other social, cultural, and economic processes that have taken place in tandem.
