The Economist's guide to valuing currencies against the dollar:
WHICH countries has the foreign-exchange market blessed with a cheap exchange rate, and which has it burdened with an expensive one? The Economist's Big Mac index, a lighthearted guide to valuing currencies, provides some clues. The index is based on the idea of purchasing-power parity (PPP), which says currencies should trade at the rate that makes the price of goods the same in each country.
So if the price of a Big Mac translated into dollars is above $3.57, its cost in America, the currency is dear; if it is below that benchmark, it is cheap. A Big Mac in China is half the cost of one in America, and other Asian currencies look similarly undervalued. At the other end of the scale, many European currencies look uncompetitive. But the British pound, which was more than 25% overvalued a year ago, is now near fair value.