esperanza, have you yet tried Eddie's mention of
http://www.Movelia.es ?
It's a kind of conglomeration of cooperating bus companies - like an "all in one" routes/prices/reservations site. Not ALL bus companies are represented though but they seem to show the exact prices that the individual bus companies show on their own sites too so that's encouraging. Check it out!
Have a nice Sunday, everyone! Saludos, MadridMan
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