I once argued with a clerk at the grocery store in my college town. I had four cases of Coke (12 packs), each of which were $2.99. She "kindly" pointed out that the 24-packs were "on sale" for $6.99 that week.
I see it all the time when you have multiple sizes of product. This is why grocers are required to put a cost per unit size statement on the shelf.