Ok, here goes.
1) Porto: I stayed at the national youth hostel; it doesn't have a specific name, in Portuguese, they are called Pousada de Juventudes, and there is one in every major city and some minor cities. Here is the website: http://www.pousadasjuventude.pt/
It has all the information, prices, and you can book online. I only stayed at the one in Porto but it was a good experience. And as you can see, these hostels are very cheap; it would be less than 10 US a person. I know the one in Porto had double rooms so you wouldn't have to be in a dorm. The only downside of this hostel is that it is out a bit; it is right along the river but a bit of a hike to the main part of town: I did it in about 45 minutes. Most of the time though, I took a bus; it stops right outside the hostel and costs less than a euro and will take you right to the center and it runs back and forth 24 hours/day I think.
2) Lisbon I can't help you out with because I stayed with the family of a friend. Sorry....
3) Lagos-I stayed at a pension type place right in the main square overlooking the ocean. It was above a jewelry shop with no distinguishable sign or anything. I was approached by a woman named Lucinda upon getting off the bus from Seville who said she had places to stay and it turned out to be completely legit and wonderful. It was 17/euro a night at the time (July) but had the most incredible view. I know there is a Pousada de Juventude in Lagos but the most popular thing for lodging doesn't seem to be hostels; most people I met were staying in pensions like myself. In fact I didn't really see hostel signs. I think you could easily show up there without a place to stay and find a nice place no problem, especially since high season is ending. But definitely expect to pay a little more for lodging in Lagos as it is so touristy...
4) Granada-I stayed at the yha hostel but I cannot seem to find it online. The address is Avenida Ramon y Cajal 2, E-18003. It is about a 20 minute walk to the historic section of the downtown. This hostel was alright but I have a feeling you and your girlfriend would want something else. As far as I could tell, all the rooms were 3 beds, so you'd likely be in with a random person unless you were lucky. Like Lagos, I didn't see many "hostel" signs but lots of pension/room signs, especially closer to the area of the cathedral and the Alhambra, so you might be better looking there.
4) Madrid-I stayed at Ole International Hostel. This is a really good location; it is close to Gran Via and right near several metro stops. The hostel was a bit cramped though, the main room and kitchen were tiny and you were always having to practically walk over people. And actually though, now that I think about it all the rooms were dorms with at least 4 people but often 12 people in them, so you probably wouldn't want this either. There were no double rooms. So I guess I can't help you here either. Of course though, there are so many places to stay in Madrid it is unreal, and lots of places in the same general area, around Gran Via, so I'm sure you could find a double room somewhere. Again, I think you could show up and look the place over first and not have a reservation. I would expect to pay at least 40/euro/night for the two of you though, since it's Madrid.
Hope this sort of helps, but I have a feeling it doesn't since I was in a different situation; traveling by myself and wanting to be in a room with others as opposed to a double room with a significant other, but hey I tried
Bye for now,