GOG originally had prices only in USD, with the same price world-wide. A year ago or do, though, they decided that introducing regional prices offered a much greater chance of seeing more games here -- primarily "AAA games" were mentioned -- and so they introduced "fair pricing" to replace "one world, one price". "Fair pricing" currently means that for some games, if you pay more than the US price (even if you change to USD, you'll still be paying your local price, just in a different currency), you get the difference back in a kind of store wallet