So I'll be in Spain from May 16-23. As of now i am planning on staying in Madrid the entire time however i could be talked into splitting my time up between two or more cities.

I want to try and keep my intra-spain travel and accommodation costs down. I don't want to spend more than $500USD. Anyway, I am open to renting a room in someone's house or a cheap B&B or hotel. However where ever I stay I would like it to be clean and safe.

Any ides or suggestions?