At present, you cannot transfer copies of Diablo III from one account to another, and it's not possible to buy Diablo III as a gift for another account, so your plan to create a US account and purchase Diablo III for your existing european account will not work.
Also, I'm not it all sure it's your account that determines your purchase price...I'm fairly certain you'd need to use a US proxy server in order to purchase Diablo III at the US rate, as Blizzard is likely filtering IP addresses to determine which region you are in, and connect you to the store for your region.
I'm not sure whether you'd have access to the European real-money store. Blizzard has made it clear that, while you have access to all regions' servers, you have access to the real money store in only one region. That could be based on the region that you purchased the game in, or the region that your current IP address maps to, in the latter case, you'd be gold if you could just use a US proxy once to buy the game, in the former, you'd only have access to the gold auction house on your local servers.