Not worth it at the dollar store: foam cups
Stocking your office? Having a brunch party? You'll need some foam cups, of course. Many people shop for party supplies at dollar stores because you can get all of your paper supplies in one place and usually in quantities that you can deal with for a small gathering. For this purpose, you might want just 50 plates, rather than 350, and if you're going to pay a premium for a small quantity, you think you're better off at the dollar store.
But if you stock up, you can save. At the dollar store, you can get a package of 45 - 7 oz. foam cups at the dollar store for $1 or .022 per cup or shop at Costco where you can go big and go home with 1,000 foam cups for $12.18 or .012 per cup.