If you read any consumer-tips article on the theme “Getting the most out of your budget,” at some point it'll probably tell you to “check the unit price” or “calculate the total cost” or words to that effect, and the moral of such stories is usually “Paying more money upfront saves money in the long run.”
For example, here's something I wrote last February:
“It's easy to forget that the lowest price or sale price isn't always the best price. … assuming two items of equal quality, which is the better deal — paying $1.00 for one ounce, or $3.79 for six?
“The dollar option certainly takes less money out of your pocket — at least for now. But what is the actual unit cost? You can pay a dollar an ounce for the first option — or pay only 64 cents an ounce, if you shell out $3.79 all at once. In the long term, the more expensive purchase actually offers the better deal — assuming you're going to buy and use the whole six ounces anyway. For things like soap and toothpaste, you definitely will.”
Despite how easy it is to find similar real-world examples, you still need to check the unit price every time, rather than assume “The bigger six-ounce option probably offers a better price than the little one- or two-ounce package.” To illustrate, check out this photo I snapped the other day, when my significant other and I visited the taco/burrito chain restaurant Moe's Southwest Grill.
If you want to buy queso or guacamole, you have three choices: a “side” for 99 cents, “cup“ for $3.49 or “bowl” for $5.99 (plus any applicable sales taxes).
How do those unit prices break down? It says there on the menu board: two ounces in a side, six in a cup and 12 in a bowl.
So if you want six ounces, you can pay $3.49 for a “cup,” but it's cheaper to pay $2.97 for three “sides.” If you want 12 ounces, even the “bowl” costs slightly more than six two-ounce sides: $5.99 versus $5.94.
Buying in bulk is usually cheaper than buying in smaller increments but that's not guaranteed, which is why consumer-tip writers always tell you: “Check the unit price.”