An item in the Post this morning and the sight of a sale on canned tomatoes reminds me of a fact, or a series of facts, interlinked. Stereotypically the grocery stores in poorer urban neighborhoods are small and pricey. The urban poor typically do not have cars, being more reliant on public transport. (Even in Reston I often see people walking from the local Safeway to the once-subsidized housing complex carrying a couple bags of groceries.) The poor live from month to month. All of which makes it difficult to take advantage of sales at stores, to invest money in food which you may eat in 6 months, rather than 6 hours.
My wife and I will stock up on canned tomatoes on our weekly shopping trip, and thereby save about 40 percent on the cost. It's easy because we have the money, we have the car (I normally walk except for the weekly trip), and the store is handy. It's taking advantage of an opportunity (which we can do on other staples), not a determined effort to limit food expenditures to a budget figure, but it does mean our food costs are lower than for a poorer couple in different circumstances.