I have stayed at:
* Hotel Alimara: decent quality hotel with nice prices (last time we went, there was a promotion that gave us a very nice suite with breakfast delivered to the room for a very low price); however, it is situated in a very isolated location (very few restaurants nearby, etc.). There's a Metro station extremely close (less than 100m.) that will take you to the city center in 10-20 minutes.
* NH Cóndor: another cheap but nice NH hotel, situated in a nice area (lots of trendy bars and restaurants); there's a train station nearby- getting to the city center will take about 5-12 minutes.