The Big Mac index is an index that was published for the first time in the magazine The Economist on September 6th, 1986 and gives an indication of the price level in a country. The index is built around the fact that a McDonald’s Big Mac hamburger is a product that is available all over the world, is globally uniform and at the same time domestically produced. By dividing the cost of a Big Mac in the USA with the cost of a Big Mac in any other country converted to US dollars you get an indication of whether that country’s currency is over or under valued towards the dollar.
In much the same way, the Famous Grouse whiskey is a globally available and uniform product and it’s price converted to cost per liter in Euros gives a (strong) indication of the general price level at the airport tax free store you are currently in. €10 is the index. Above that = expensive. Below that = go ahead!