"Cheaper" By the Bundle

What's wrong with this picture?

I'm used to kvetching about the (mis)use of language in a post-literate society, but as an English major by training (and trade), I'm not used to having to do the math for people.

Then again, I remember an article I read some years back about "weasel" words and phrases in advertising. It brought up the issue of unfinished or ungrounded comparisons--a typical trick in advertising. It suggested that whenever you see or hear an ad with a comparative--quieter, faster, cheaper--you train yourself to finish it by adding "than what?"

Great gifts are "cheaper" by the bundle. But getting a bundle of four doesn't make them cheaper by unit or overall than getting a bundle of two. Cheaper than buying each of the four books individually, I guess. Not cheaper than if you had bought all four books in sets of two by two, though.