hello.
what is the cheapest way to do this journey?

I've been investigating this, too. Bus is cheaper than train--only about 13 euros one-way. This website has the bus info:

www.continental-auto.es

Buses are also more flexible; there are more buses than trains making the trip daily.

I would definitely recommend the bus as well, as the previous poster stated, they're more flexible with more departures, usually take less time, and are cheaper!

Buena suerte!

-GG

BUS! I did this route numerous times last semester, and it is by far the cheapest and very easy. The buses are nice, and they run almost one every hour, all day and night.
Have fun!
Kelly