Study: Walmart stores jack their prices up in poorer neighborhoods
Of seventeen choices, nine grocery items had a different price depending on the store. The price difference favored the Folsom store, an affluent suburb, over the Rancho Cordova and South Sacramento stores, non-affluent sections of Sacramento county.
That is, the stores in poor neighborhoods had higher prices, not the other way around. Perhaps the customers in more affluent neighborhoods had better access to alternative grocers, or perhaps the poor neighborhoods were more expensive to operate a store, necessitating premium prices.
I may not be able to guess the reason for price variances based on this comparison and my stab at demographics, but I'm happy to know about it. I feel like I figured out a secret.