These threads always bring out posters who are proud of how little they spend or how much they spend. Most of us who have been here a while have a pretty good idea of how much things cost, and there are few if any areas where overall expenses (excluding housing) can drastically be kept below average costs for Thailand.
When someone posts they only spend 2,500 baht a month on food, chances are they are either eating like a bird (and perhaps drinking like a fish) or the quality of their diet is being compromised (highly processed, high sodium, few anti-oxidants, low fiber, low in fresh fruits and vegetables, etcetera. Someone claiming to spend 30,000 for one person on food is probably combining groceries with eating out expenses, eats enormous amounts of food, wastes enormous amounts of food, shopping at high end grocery stores, and undoubtably eating a lot of imported food.
Record keeping is another issue. People say they only spend 2,500 a month on food, while forgetting all the other months where they spent much more. It is also true enough that many, if not most, locals live on less than 20,000 per month (excluding rent), but when you start getting into the weeds and looking at their quality of diet, medical and dental care, savings for retirement, wardrobe, ability to finance children's higher education, ability to own a motor vehicle, etc., it quickly becomes apparent that significant compromises in lifestyle and standard of living are being made out of necessity, not by choice.