American, Delta and United generally do not offer low one way ticket prices to Europe. An interesting development happening now is Star Alliance airlines have low cost one way tickets for San Francisco-Barcelona at $162 to $180 for travel on Air Canada, Lufthansa, Swiss and United Airlines for travel January to March, then late September-October 2019.
The launch of Level Airlines marketed as Iberia flights and its affiliation with Avios frequent flyer points gives a competitive edge to Oneworld Alliance for their nonstop San Francisco-Barcelona route. These low one way fares with Star Alliance airlines look like an attack on Level Airlines.
Still, if you want to fly Star Alliance both ways across the Atlantic, then you need to book a round trip ticket. Star Alliance ticket prices for Barcelona-San Francisco one way are still priced higher than a round trip ticket.
Or fly Level Airlines nonstop back to SFO.
United Airlines San Francisco SFO – Newark EWR – Barcelona BCN
Lufthansa Airlines San Francisco SFO – Munich MUC – Barcelona BCN
An interesting feature of this sale are the lowest priced round trip tickets around $350 require a trip of 7 days or less. Stay more than 7 days and the price rises to mid-$400s to mid-$500s.
Lufthansa ticket for Swiss Airlines flights
San Francisco SFO – Zurich ZRH – Barcelona BCN – Zurich ZRH – SFO
Lower ticket prices are available with Star Alliance flights originating in Barcelona to SFO in low $300s round trip and the length of trip can be short or months long.
Lufthansa Airlines Barcelona BCN – Frankfurt FRA – San Francisco SFO – Zurich ZRH – Barcelona BCN