Baltimore, MD (BWI) to Barcelona, Spain (BCN).  $394 round-trip, including all taxes. Travel on select dates through the end of March.

Flying to Barcelona continues to be one of the cheapest ways to visit Europe this year, but there aren't that many sub $400 fares that we've come across until we found this one.

For a wider travel window dates can be found at the $409 or $419 price range, but will often require two connections on one of the travel legs. Easiest to book using Delta.com