American Airlines/ Iberia and SkyTeam have cities across Spain and Porto, Portugal priced in the upper $300s round trip for winter and late fall travel, low $400s in April-June, low $500s July-August and back down to $300s by late Oct. These low fare deals include many of the Spanish resort islands, including Tenerife in the Canary Islands, northwestern Spain, like Vigo and Santiago de Compostela, and even Porto, Portugal.
SkyTeam matches fares to many of these cities. Star Alliance is low fare to Madrid.
Google Flights Sun March 5 – Mon March 13
Oneworld filter shows 8 cities in Spain and Portugal priced in $300s round trip.
There are several other airports too.
Iberia Airlines New York JFK – Madrid MAD – Palma Mallorca PMI – Madrid – New York JFK