Spain's a big (European) country so the easiest way to travel between major cities is usually to fly and there are good deals that can make that cheaper than train.
vueling.com is a Spanish no-frills operator, BCN-based, which has cheap deals but often at inconvenient times. You can get as good a deal and far more flight options often on the other Spanish airlines. On Spanair.com the lowest webfare is 19€, Iberia's also under 30€; Aireuropa will be competitive. Plus taxes in all cases of course. These really cheap deals are often available even booking only a couple of weeks in advance, if you are flexible with departure times (mid-morning or mid-afternoon often cheaper as non-business travel).
For example, i have just booked BCN-MAD return for 19-25th June with Spanair for 113€ pp total; Iberia wd have been 135€. OK, so they're not quite ryanair's rockbottom prices (but ryanair's aren't always rockbottom either!) but still a good deal (in Feb on a vueling superdeal we got to Lisbon and back for 54€ each).