Delta, Air France, and KLM have round-trip flights from Seattle (SEA) to Valencia or Porto, Portugal (VLC, OPO) for $559-$579 with a connection. Flights February to April. Extended weekends available.
Air France and KLM have round-trip flights from Los Angeles (LAX) to Valencia (VLC) for $460-$470 with a connection. Flights February, March, October, and November. Departures on Monday, Tuesday, Wednesday, and Sunday. 7-day minimum stay required.
Air Europa has round-trip flights from New York/Newark (JFK/EWR) to Valencia or Malaga (VLC, AGP) for $404-$414 with a connection. Valencia flights July to November. Malaga flights in November only. Origin/Destination reversible.
Note: Some small OTAs on Skyscanner have this for about $375. If you plan on booking there, beware of any site that charges a fee to cancel a ticket within the 24-hour “free” period. Use unknown booking sites at your own risk.