Canned goods are probably the cheapest food there is, and it’s cheap even in the regular supermarkets. To put things in perspective, you can find canned kidney beans cost for as little as 79 cents. So why would you pay $1 for them in dollar stores?
Dollar stores try to keep things simple, and they put a $1 tag on everything. You will pay that price for items that cost them 99 cents and things that cost them 15 cents. As a result, canned food is often cheaper in regular supermarkets.