I Found This An Interesting Read,

how dollar stores, or pound shops, as we call them in the UK,

make a profit, the article is from HowStuffWorks, soprices and comparisons are in dollars, but the same will hold true whatever your currency is, from the article, photograph John Sposato,

 According to analysis by The Guardian and The Washington Post, paying $1 for a 16-oz bottle of milk at a dollar store is the equivalent of paying $8 for a full gallon, which is more expensive than the fancy organic stuff at Whole Foods. Similarly, the $1 bag of raisins at the dollar store weighs only 4.5 ounces (128 grams), while a 72-ounce (2-kilogram) bag from a big-box store costs $10.50, or 52 percent less per ounce.

By shrinking package sizes, dollar stores can get away with charging way more per volume, which is one of the main strategies that dollar stores use to wring the most profit out of every sale. Just because you only paid $1 for that roll of aluminum foil doesn't mean that it's a good deal. That $1 roll is only 15 feet (4.5 meters) long, while Walmart sells a 75-foot (23-meter) roll for $4.06, the equivalent of more than 18 feet per dollar. (Of course, if finances are tight, maybe you'd rather pay $1 for 15 feet of foil rather than $4.06 for 75 feet.)

That's a part of what's known as the "poor tax." We know it would be cheaper to buy in bulk, but that's not possible if we don't have enough cash to do it, so now I know how they survive at a dollar an item, or in the UK a pound.