One person I polled replied with something like "[laugh] have any of these people been to a place where you can buy cheap bulk food [that isn't one one of the expensive membership places like Costco]? Look at the SES of the shoppers!"
Well, there's that one guy I know who owns two planes who buys rice by the pallet, but that's really unusual. I suspect no one who's saying that poor people pay more for food than rich people know someone who does this.