it's not just based on what operating costs run for a grocery store-it's also what they can get from consumers.
we live in a town (very minute low income population) that is in between 2 much larger towns-for decades the only grocery options here were a small privatly owned store which catered largely to the hispanic population carrying alot of certain types of meats and products-very pricey in my opinion, and a safeway store. while the safeway ran the same specials as the safeway's in the larger towns-the rest of the items were as much as twice as costly as they were in their other town counter-parts. we had a super-
walmart go in last year, and surprise surprise-safeway's prices are now the same as they are in the larger towns. they are no longer 'the only game in town' and have to offer lower prices to keep the customers coming in (they are still way more expensive than super walmart-but much more in keeping with their counter-parts).
there are def. bsns. that take advantage of low income people and that makes it harder on them as consumers, but i have to say that in alot of the cases i handled in social services the worst enemies economicaly the clients had were themselves. i worked in a region of the san francisco bay area that had some of the richest and poorest residents. to try to help people stretch their meager welfare checks/food stamps we did alot of counseling on budgeting and economics. talked to clients about comparing prices and trying to get the best value for the money-many had the mindset of 'i won't go to 'x' store-i ONLY shop at 'y' (the realy high end 'trendy' grocery store that was a good distance away from them in the wealthier end of town), 'pack my lunch for work?-no-way, i NEED my 'lunch dates' every day with the 'girls'' (paid more for lunch and their starbucks than they made each day

), 'drop all the premium channels and pay per views-NO WAY!". they opted out of free checking/savings accounts (with no fees for atm use) at large national banks that had special programs for our clients because they did'nt want to be 'seen' at that bank. it realy got frustrating when you tried to explain that if they were on a program that provided funding for a limited period of time after which they had to pick up the full cost (like child care or housing subsidies that helped for 6 months until their probation period at a job was passed and they were making a decent wage) it was best to go with a child care program or house that their anticipated wage could cover-nope, they opted most times to go with the 'fast food' high end child care center whose monthly fee was over 50% of their gross, and the house that once their subsidy ended would rent at twice their total monthly income. the attitude was 'i want it now, i'll worry about it when it happens'
the truly successful folks were those that took the information we provided and went running with it-i had families that realized what bulk buying power could do for them, so they all kicked in and got a costco membership. they'de get together and figure out what they needed grocery wise, bought it bulk and split it up. if someone was making a trip in their car to an adjacent area that had some good stores for getting deals at (like the same chain store as in their area but better prices)-everyone kept a running list of items they would have that person pick up (diapers-insanely expensive at their local store could be bought at half the cost down the road). while some of these people might never get 'off the system' due to health or other long term issues they were able to maintain a decent quality of life.
dh and i are looking at moving in the next year, we joke that if we want to see what the true social/economic dynamics of a community are we have only to look at the local yellow pages-if there is a proliferation of check cashing stores, pawn shops, 'rent to own's', and motels that offer weekly and monthly rates we know it's likely a much lower standard of living.