I bought a one way BCN-MAD last Saturday afternoon. The terminal was deserted. Coach costs € 153 plus € 20 issuing fee. IB flies every hour, at peak times every half hour, but Saturday afternoon there was nothing for three hours. The lady at the shuttle desk suggested I try the regular flight from Terminal 2 in one hour, the shuttle ticket would be valid. Since I had time I tried Spanair at the far end of Terminal 2, which quoted me exactly the same fare as IB, but only offered the next flight in 4 hours. I did not try Air Europe since they had no flights at all on the Saturday afternoon/evening. I eventually got on an IB flight coming from STR, half full, bit cramped seating in the last row, but ok for a 4o min flight
However, I saw tickets advertidsed for as low as € 25 on both IB and JK. Their websites are quite good, so try them if you can book in advance.