> If a TV adverts costs £X but raises 2X, is that a sensible cost? Maybe, but quite possibly not, because that 2X didn't magically appear, it came out of other

> If a TV adverts costs £X but raises 2X, is that a sensible cost? Maybe, but quite possibly not, because that 2X didn't magically appear, it came out of other people's pockets, and you've got to properly account for that as a negative impact you're having.