United Airlines with Star Alliance have flights to Portugal on sale from about $433-$450 round trip from Los Angeles, San Francisco and San Jose airports. These are ticket prices through United.com.
Ticket prices were only $3 to $10 discounts through Momondo.com searches for dates I compared.
Lisbon is on sale from all three California airports and Porto is discounted for LAX and SFO flights.
United Airlines Los Angeles LAX – London LHR (United) – Lisbon LIS (TAP Portugal) – IAD (UA) – LAX
($433 tickets are Air Canada flights).
United Airlines with Swiss Airlines and Air Canada partner flights
San Francisco SFO – London LHR – Geneva GVA – Porto OPO – Toronto YYZ – SFO