I've been watching this fare for about a week. The price quoted is available through Lufthansa's website only. I can see flights right through to August 2018, so it's quite attractive. Also possible ARN-MIA for $316, ARN-JFK for $278 and ARN-LAX for $291.
This books into K fare, which earns no miles in many FFPs. Also beware, I believe this fare is with no checked baggage allowance.
A cheap way to fly across the pond, but that's about it.