Thanks @cryptostar! All prices are Volume Weighted Average prices. If USD Fiat currencies exist for a currency, the Volume Weighted Average USD price is used for further calculation. e.g.: There are several markets where ETH/USD is traded. For the calculation of GNT/ETH the GNT USD value is calculated on the basis of the Volume Weighted Average of all ETH/USD markets, and so on.
It is a very good question, and I should publish some explanation on how prices are calculated in the future.
Thanks for the feedback!