Fly nonstop from Boston to Barcelona, Spain for $264 roundtrip on Iberia's low-cost subsidiary Level. You'll have to book two separate one-way flights in order to get this price on a few fall and winter travel dates.
For example, on Orbitz, I found seats from Boston BOS to Barcelona BCN on Tuesday, October 29 for $149 one-way. For your return flight, I found a one-way trip from Barcelona BCN to Boston BOS on Thursday, November 7 for $115. That's a total of $264 roundtrip. It's easiest to book via a third-party site to find this price point.
You'll find these fares available on a Tuesdays, Thursdays, and select Saturdays from October 29 through the end of January 2020.
This price only includes one standard sized carry-on bag plus one personal item. Checked bag fees start at $44 each way. Meal service and advanced seat selection are also not included in this fare. Level sells bundled fares if you need more options.
Need a room in Barcelona? Here's what's available for the above dates on Hotelwatchdog.