If US dollar stores are anything like UK pound stores, they are vastly more expensive for food than the discount supermarkets -- Aldi, Lidl, Iceland and similar. They're OK to great value for many non-food things, like a cheap spare pair of reading glasses, notebook, pencils or USB cable etc. If there's a pound store it's fairly safe bet one of the above three supermarkets will have an outlet not too far away too.
Nearly all the food they sell is a custom, pound store only size, and the price per gram or litre is pathetic for anyone lacking funds. They trade on appearing to offer price advantage, perhaps 10p or 20p in the pound cheaper than the supermarket, by cutting 25 or 35% of the product. If you aren't comparing price/unit you're basically getting scammed by poundstores on all food and drink. Of course they appeal most to the section of society most uncomfortable with maths by seeming cheap, even if the truth is on the small print of the shelf label.
They positively drip with unhealthy only options. Miles of chocolate, cereal, milk shake, soft drinks, tinned beans. No veg, no rice, no pasta, pulses, low sugar soft drinks, milk itself.