How are currency prices determined?  

Currency prices are affected by a variety of economic and political conditions, the most important of which are interest rates, inflation and political stability. Moreover, governments sometimes participate in the forex market to influence the value of their currencies, either by flooding the market with their domestic currency in an attempt to lower the price, or conversely buying in order to raise the price. This is known as central bank intervention. Any of these factors, as well as large market orders, can cause high volatility in currency prices. However, the size and volume of the forex market makes it virtually impossible for any one entity to "drive" the market for any length of time.