It makes sense that you'd get a lower cost-per-item price when buying in volume, but sometimes that isn't the case. Money-saving food blog Pay Less for Food points out that supermarkets are aware that you think this way and will sometimes raise prices for bulk items as a result.
If you're buying in bulk, be sure to do the maths before you get to the checkout lane to avoid overspending. It may only mean minimal savings for a single item, but that can add up quickly if you buy high quantities of most items. (Stores in Australia are obliged to display unit pricing, so the comparison is pretty easy to make.)
The Sneakiest Grocery Store Trick Yet? [Pay Less for Food]