Hi, I'm also looking into accomodation in Córdoba and Málaga during that period.
If you want really cheap, youth hostels are a good option. (
www.reaj.com)
However, in most but not all cities and towns I have found the cheapest places just by walking around and asking in pensions and hostales.
single rooms/ night (shared bathroom, no aircon etc) during peak season in July -August 2005:
Sevilla 13, Salobreña 12, Madrid 15 Euro et c.
If you want to book ahead via internet it's hard to find that kind of places.
I try to go to the tourist information to get a map/list with pensions and their rates.