You see something advertised as so many pounds or dollars or per cent off the original price. Does that fact make any difference whatsoever to your decision to purchase it?
Unless you are a complete alien to that shopping environment then the answer is clearly no. The offer should make bugger-all difference to your decision to purchase. When you are shopping then the two main factors are "I have this much time and money to spend" and "I have this much value to obtain". The third factor of "this is how much I have 'saved' versus some totally notional figure made up by the retailer" is completely irrelevant.
Nobody has the complete information of free market economic theory. But we generally have enough information to decide whether something is worth buying.
I know the things stated here are sometimes very obvious. But then I look around and see people who really do seem to choose on the basis of what they think they are saving instead of what they think they are buying. When describing a purchase some people really do glory in how much they have "saved".
There is no absolute line, we all take many factors into account. But some are just lunatic.