Delta and SkyTeam airlines have many cities in Spain on sale from $382 to low $400s round trip beginning mid-August into late fall months. Madrid, Bilbao, Valencia, and island airports like Ibiza and Palma de Majorca are some of the low fare destinations.
Many of these flights show up at low fares on Google Flights and then change to far higher prices when dates selected.
Use Google Flights to find low fare dates, then switch to Momondo.com for online travel agency sites with these low fares in $300s and $400s. I also found some of the low fares directly on KLM.
Alicante ALC $414-$444 Aug-Nov
Bilbao BIO $398-$416 Aug-Nov
Ibiza IBZ $395-$433 Aug-Sep
Madrid MAD $382-$387 Aug-March 2018
Palma, Majorca PMI $395-$421 Aug-Oct (not many dates)
Tenerife, Canary Islands TFN $537-$555 Sep-Oct (few dates)
Valencia VLC $415-$435 Aug-Dec
Vigo VGO $401 Aug-Sep (few dates)
KLM Airlines Los Angeles LAX – Atlanta ATL – Madrid MAD – Ibiza IBZ – Amsterdam AMS – San Francisco SFO – LAX