Go to www.iberia.com. I typed in the date you want to fly one way from Madrid to Barcelona and the cost is 33.45 Euros or approx $38 US dollars. Flight takes 1hr and 10 minutes. Beats the travel time and cost by train.

If you get an E-ticket it is even cheaper, 4 Euros less.

I am travelling around Spain in September and October and have already booked flights within Spain with Iberia from this web site. (Barcelona to Seville, Madrid to Bilbao)