I walked the Camino Portuguese from Lisbon in May and just now (slowly) getting my journal typed up - so I'm reminded of all this again. After Santiago I bussed back to Lisbon for 2 tourist days before flying back to U.S.. Because I could only get a bed for one night at a time, I stayed in two hostels in two nights (both in a multi-bed dorm room). Both under 20 Euros. Take the Baixa - Chiado metro stop. The metro is very easy to navigate. Both are a 10 minute walk to Cathedral Se.
The first one is my recommendation: Lisbon Lounge hostel. Everything about it was nice and roomy, plus the breakfast was great! Having been on the Camino for a month, I literally could not remember the last time I just sat down for an enjoyable hot breakfast! The second hostel shall remain unnamed (but whose initials are GN) and was downtown as well. Particle board bed, lumpy (yes - actually lumpy) mattress, weak wi-fi, not as good breakfast as LLH, and smaller dorm rooms.
I'm no hostel expert, but this was my experience. Good luck and Bom Caminho.
PS. Before Point de Lima, stay at
Casa da Fernanda (call at least 2 days ahead to reserve). In Rubiaes, stay at O Ninho (not Escola muni). Both offer a "Caminogasm" experience.