I had the opposite experience. I suspect people spending time abroad temporarily, but long enough to go grocery shopping, tend to arrive at the most expensive grocery stores during their temporary stay.
I'm not talking about Wholefoods, that's overpriced here as well. I'm talking about a slightly grubby supermarket in an unfashionable part of Brooklyn about 5 years ago. Maybe food prices vary a lot across the US, they don't really in the UK as the big 4 supermarkets all advertise their offers nationally. Smaller supermarkets can be more expensive, but an extra 20% not 300%.