United and Air Canada have tickets priced from $380 round trip for San Francisco SFO to Lisbon LIS. This is a seldom seen low fare to Portugal that has been around periodically over the past month. Portugal has rarely been on sale from California in the past two years, especially compared to the numerous sales to Barcelona and Madrid.
Travel departures at lowest prices start in mid-September through early December, then a few dates in early January.
Air Canada flights are as low as $380 round trip. United starts around $390.
United Airlines San Francisco SFO – London LHR – Lisbon LIS – Washington IAD – SFO