The entrance for the first six floors of the building is free, so don't hesitate to enter and take a look. The large cultural space inside is called CentroCentro (http://www.centrocentro.org/), and it will probably intrigue you too.
Anyway, if you care to enjoy the superb view from the rooftop terrace & bar, you'll have to pay the €6 entrance fee.
Open 10:00 - 20:00. Closed Mon.