You're both sort of right really. In theoretical terms, Alc is right in that you can't assign a value to things like prestige and statement of intent and look at how much that has generated for United. That's a bit of a correlation causation thing.
Thing is, that's exactly how it works in reality exactly because you can't measure it. Pretty much every marketing department works on implied value and a huge base of assumptions.
Re: Pogba, signing him would only boost the value of the club. Them being on the stock market won't change the fact that United have acquired a massively marketable asset for a rate they can afford. That's what people don't seem to get, it's all relative. It's insane money, but if you just look at United's raw figures, it's the equivalent of other clubs spending a **** of a lot less.