Huespedes Jaén,
www.hjaen.com has a nice 3 bedroom apartment at 100 euros for 5 people. One double bedroom, one single, and a twin, plus a couch in the living room.
We stayed there in June 2003, liked it, and reviewed it. Good location, airy, nice folks, quiet yet easy stroll to most places. Down: wimpy A/C and tiny slow elevator.