You have have had responses to your previous query posts, which vary very little from this one - why not read them?
If you have several weeks before you need to travel then the advance discounted PROMO fares will give you a cheaper ticket. The normal one way Flexible ticket price is around €55 not bad considering. Usually the cheapest Promo is going to be below €30 but you may have to catch an early or late train outside the normally busy periods.
You do get very cheap tickets using the ALSA bus but they come with conditions like no refunds if cancelled but as low as €5 who cares ? Journey takes around 4 hours but some buses go to the airport or other stops.
In case you cannot find your prior question, here it is:
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