You are very well placed in Porto given the number of low-cost routes operated from there by Ryanair.com and others.
Here are the low-cost destinations served from Porto by Ryanair.com, Easyjet.com and Transavia.com, and Aigle Azur. (Note some are seasonal.)
Ryanair.com
Belgium
Brussels Charleroi
France
Bordeaux
Carcassonne
Clermont Ferrand
Dole
La Rochelle
Lille
Marseille
Paris Beauvais
Paris Vatry
St Etienne
Strasbourg
Tours
Germany
Baden-Baden
Bremen
Dortmund
Dusseldorf Weeze
Frankfurt Hahn
Hamburg- Lubeck
Memmingen
Nuremberg
Italy
Milan Bergamo
Rome Campino
Netherlands
Eindhoven
Maastricht
Portugal
Faro
Spain
Barcelona El Prat
Palma de Mallorca
Tenerife Sth
Easyjet.com
France
Toulouse
Transavia.com
France
Paris
Nantes
Portugal
The Netherlands
Aigle-Azur.com
France
Paris
