Our price is an average price retrieved from coinmarketcap.com and worldcoinindex.com and weighted by volume of transactions in the exchanges.
This is important for the correct tax calculation since every transaction is taken into account with this asset value at the time of execution. Our system calculates the tax and gain based on these values.
Currently you can choose to override our average price by retrieving the BTC price from Bitcoin.de and the USD price from Bitstamp or CampBX. Overriding the price source for other coins is not possible at the moment.