Buy one, get one free

"Buy one, get one free" or "two for the price of one" is a common form of sales promotion. Economist Alex Tabarrok has argued that the success of this promotion lies in the fact that the price actually takes into account the fact that two items are being sold. The price of "one" is somewhat nominal and is typically raised when used as part of a buy one get one free deal. Whilst the cost per item is proportionately cheaper than if bought on its own, it is not actually half price.
Read article on Wikipedia