The exceptionally affordable fares, which were first spotted by the folks at Scott's Cheap Flights, are available from Boston and New York to cities Berlin, Frankfurt, and Munich. At the moment, the cheapest flights appear to be for travel dates between October and January 2018, but that may have changed by the time you read this.
As of this writing, the cheapest roundtrips on Google Flights were in December on Wow Air, departing both Boston and Newark for Frankfurt at just $300 a pop. However, there are also a number of very good December fares on Icelandair in the high $300s from both Newark and Boston to Berlin. In both cases, there's a layover in Iceland, but that's not so bad, especially if you feel like exploring the Nordic island nation by taking advantage of the either airline's spectacular stopover promotions. SCF reported that there were also a number of great fares to Germany during this same timeframe from Seattle, Denver, and Washington, D.C., but those appear to have disappeared (at least, for now).