Hi, tmlsholt,
I have always found that one way car rentals are extremely expensive in Europe. Thinking that maybe things have changed, I just tried what Albertinho suggested using orbitz to get a sense of car operators. I found lots of cheap daily rates, but the one way drop-off fee pushed the total to well over $1000 US.
So, Albertinho, if you have some tricks up your sleeve about how to get that fare down, let me know!
I have taken the bus from Santiago to Lisbon, it is a dreary all day affair with a 45 minute stop in an awful highway cafeteria. I think there is just one bus a day leaving Santiago around noon, but you can easily find that information on the web.
The train involves a change in Vigo, I have never done that. But if I had to make the trip again, I would go for the train. Not that there was anything specially wrong with the bus, it's just long and boring and you are trapped in your seat. Buen camino, Laurie