Lufthansa and Brussels Airlines have flights from Dallas (DFW) to Berlin (TXL) for $361-$432. Flights February to May. 7-day minimum stay required. Flight may operate as a United code-share.
Note: Fare allows for a free carry-on bag, but checked bags will cost about +$60 each-way.
Note: Flights to Berlin have been around $400 since last October.