Air Canada and Star Alliance airlines have Los Angeles to Barcelona priced as low as $305 round trip for travel in November. Ticket prices are as low as $323 for travel in August. Some dates searched on Google Flights show low price itineraries, but no direct links. Google Flights will lead you to the low fare dates, however, you may need to go to Skyscanner or Momondo to find tickets for many of these dates.
Here is one sample fare that showed as $332 on Google Flights, then priced at $305 round trip on Air Canada.
Air Canada Los Angeles LAX – Toronto YYZ – Barcelona BCN – Montreal YUL – LAX