Virtual economies are usually pretty contained and the currency very controlled. Users drive activity in virtual economies, but in many of the implementations, the things that people can buy with virtual currency are controlled by the provider. This means that the price of virtual goods are controlled in the same way a government would control prices in a traditional command economy. Where fluctuation is common is with regards to how virtual currency is accrued. Users often earn currency through actions (which are subject to change) or buy them using real money. Most virtual currencies are tied to a real currency, but as real currencies have fluctuating exchange rates in relation to each other those rates can affect the prices of virtual currencies.