A similar deal is available, bound for Dusseldorf, if you're leaving from New York (NYC), Los Angeles (LAX), Miami (MIA) or Philadelphia (PHL). Trips on that route can be found as cheap as $351 for flights from September through November.
Like with the last deal, head to the ITA Software Matrix Airfare Search and enter the appropriate departure city code (above, next to the city name) and the outbound advanced routing code "dl+ /f bc=v." Then make the arrival city DUS with the return advanced routing code "dl+ /f bc=v."
While the flights get down to $351 a couple times over that date range, as you can see above, there are a lot of cheap flights from $400 to $450 throughout October and November. Even if you're not getting the cheapest deal available, it's still a deal.