On a budget, CentroCentro / Cibeles Palace Rooftop still earns its price.
~€4 (mirador)
A cheap, central panorama: the mirador gives sweeping views over the Cibeles fountain, Alcalá and the rooftops, and the building itself is a stunner. The catch is it's a small terrace with limited timed slots, and you queue for what's ultimately a brief view.