Hmm, never heard of Segundamano being used for teaching jobs (just for buying/selling items.) There are a couple of websites devoted to nothing but private lessons, and a Facebook group for English teachers giving away classes, but 90% of my students have come from word-of-mouth recommendations. During the summer (if you can tolerate children), a lot of people work as English camp counselors or arrange live-in situations (free housing and food in exchange for daily conversation or childcare in English.)
A furnished room in a good area is around 300-350... I was paying 250 + gastos in a 3-bedroom in a killer location (2 blocks from Iglesia.) Now I'm paying 275 for a furnished room on Lopez de Hoyos, but it's worth it to only have 1 roommate instead of the usual 2-4. Even living by yourself is not that expensive - one friend is paying 450 for a studio in La Latina, and another is paying 600 for a really nice 1-bedroom in the heart of Malasana.
P.S. Ha, as if on cue, as I was typing this, I received an e-mail from my friend/student who works at Iberia. They fired the inexperienced teacher who was giving in-company classes, and he wanted to know if I'd be interested or have any friends who might be (since the academy apparently has no one to take the class): "
Me pongo en contacto con ustedes para informarles que no les puedo confirmar por el momento el sustituto definitivo de su profesor (name) y lamentablemente no disponemos de un profesor sustituto que pueda impartir su clase de hoy, por lo que debemos cancelar dicha clase." Trust me when I say that there is more demand than supply here right now 
