On a budget, weigh it — Sé de Lisboa (Cathedral) isn't cheap for what it is.
Nave free; cloister ~€5
A stout, twin-towered Romanesque pile that has survived since the 12th century and several earthquakes — atmospheric and free to enter the nave. The catch: the interior is austere and dim, the cloister and treasury cost extra for fairly modest returns, and it photographs better outside (with a tram passing) than it rewards inside. A quick, free, worthwhile pause.