United/Star Alliance have Los Angeles or Las Vegas to Barcelona on sale from $535 to $552 Jan 9-May 10 and SkyTeam from $552 to $571 January 9-April 5. Lowest Star Alliance fares are Air Canada, but there are also options to fly all Swiss Airlines or Lufthansa Airlines for slightly higher $500s.
Las Vegas – Barcelona $552 Delta, $559 United Sat March 4 – Sun March 13..
United Airlines ticket for all Swiss Airlines operated flights:
Los Angeles LAX – Zurich ZRH – Barcelona BCN – ZRH – LAX
Delta Airlines Los Angeles LAX – New York JFK – Barcelona BCN – JFK – LAX