the cheapest flight tickets are to be found at
www.skyscanner.com
(this site can be useful:
http://www.flightconnections.com/)
Going back from Santiago de Compostela to Porto, Porto Airport, Lisbon or Algarve:
Take the RENFE-train from Santiago de Compostela to the Vigo-Guixar railway station in Vigo (€10 -> 17 train connections per day) and from the bus station in Vigo take the AUTNA bus (€12 to Porto & Porto Airport, €32 to Lisbon, €42 to Algarve, 5 buses per day). Reservation of both tickets can be made online. You can even pick your seat in the AUTNA-bus online. The AUTNA-bus has a toilet.
You can also take the ALSA-bus to Tui (= Spain) (about €20) and across the river in Valença (= Portugal) you can take the AUTNA-bus (€8,05 Porto & Porto Airport, €28,05 to Lisbon, €38,05 to Algarve, 5 buses per day)
You can also take the Castromil-bus from the bus station in Santiago de Compostela to Vigo. They have their own ticket office at the bus station in Santiago de Compostela.
From the Airport in Porto you can take the metro to all destinations in Porto.
RENFE (= trains in Spain):
http://www.renfe.com/EN/viajeros/index.html
AUTNA (long distance bus in Portugal):
http://www.autna.com/venta/index.php
Autna-tickets probably also for sale at the following address:
Viajes Zafiro Tours Sa
Montero Rios, 48
15706 Santiago de Compostela
tel.nr.: +34 (0) 981 562 952
You can also take the bus from Santiago de Compostela to Fatima in Portugal (the bus takes 8 hours)
Ask also more information at the Tourist Office, at the Pilgrim's Office and at the Bus stattion in Santiago de Compostela.
See if ALSA or Castromil can take you even to Porto. but so far the train to Vigo and the bus from Vigo sounds to me the cheapest.
Count for less than €30 p.p. to reach Porto from Santiago de Compostela! Buuen Camino!
And stay about two full days in lovely Porto. It is a beautiful World Heritage City:
for an impression about Porto see
https://www.facebook.com/media/set/?set=a.10152619122129282.1073741842.761289281&type=1&l=d85328e93a