In my opinion, although it is about 100 km longer than the most direct route, the best and most economical route from Santander, Spain to Praia da Luz, Portugal is via Valladolid, Salamanca, Merida, Seville, Ayamonte and Lagos.
Apart from 20km south of Salamanca where the road is being upgraded to motorway, this route is dual carriageway most of the way and has no tolls. Additionally, fuel is considerably cheaper in Spain, so it pays to travel in Spain as much as possible.
A Route Map is shown below:(Click on full screen to view)
santanderluz
