For example, I found seats from Boston BOS to Barcelona BCN on Friday, November 2 for $149 one-way. For your return flight, I found a one-way flight from Barcelona BCN to Boston BOS on Sunday, November 11 for $116. That's a total of $265 roundtrip.
You'll find these fares available on a few Sundays, Tuesdays, and Fridays, in late October and early November.
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.