I am walking the Portuguese route in June. What is the best way to travel from Santiago de Compostela to Lisbon for my return flight home?
bjustin, check the ALSA bus company website. In there, you will find information about the bus that goes from Santiago de Compostela to Lisbon. Every day, at 12am you have a bus departing to Lisbon. It stops in Vigo, Braga, Porto (Airport and Casa da Música), Albergaria-a-Velha, Coimbra and Pombal or Fatima (it depends from the time of the year).
If you buy the ticket with 30 or plus days in advance, you can get a 50% discount. However, if you choose this promotion, you can't change your ticket date, nor have a refund if something happens. Has I've said, the bus departs everyday at 12am from Santiago (11am in Portugal), and arrives in Oriente station (the nearest one from the airport) at 8:30pm Portuguese time.
For what I remember, you don't have flights from Santiago to Lisbon; I think that you only have them to Porto. The only Airport in Galicia that has a flight to Lisbon, it's the Airport in A Coruña, and the flight ticket costs about 120€ (flying on Portuguese Airlines, TAP).
I hope that I've helped you.
Best Regards
Diogo