The Paseo de Presa is a good suggestion. I rented a room at La Casa de Las Brujas, where I was also taking classes at the Academia Falcon. That was NOT cushy, but it was very cheap. But it is a very quiet neighborhood, and very beautiful and green. There's also a bus that runs regularly into El Centro. If I recall, it was 2 pesos, i.e., 20 cents. Something in that neighborhood. A cab ride was generally about $5, or cheaper if you had negotiating skills. Also, it's really not that far of a walk to El Centro. Maybe 15 minutes at a good pace.

Note: If you take a weeks worth of classes, you could probably also easily arrange for a host family. If you stay longer with the host family than you are taking classes, I don't think anyone will complain. (The reason I didn't go the host family route was that I knew I'd be doing a lot of late night coming and going, and I didn't want to put a host family through that.)

One other possibility is to find a quality hospadaje. My first week in Gto. I stayed at the Casa de Pita, which was both wonderful and amazingly cheap. Think of it as a quality bed and breakfast, but for $20/per person per room. I took a smaller room for $14, but I was alone and didn't spend much time in my room. It was for sleeping, showering, changing clothes, and storing my gear. I would have stayed, but my room was already booked to someone else for Semana Santa.

Oh, the web address for Casa de Pita is:

http://www.casadepita.com/