How do I add an exhaust fan to an old attic?
There are solar-powered gable fans - an Air Vent-brand fan can be picked up at Lowe's for just over $200:



The solar panel is on a cord, so it can be installed in the most sunny area while the fan sits in the gable.

From the Air Vent website:


  The Gable-Mount Solar Vent is an exciting alternative to traditional attic ventilation. That's because it is powered by a solar panel that collects energy directly from the sun and converts it into electricity -- naturally -- to power a durable, high efficiency 24-volt DC motor. As a totally solar-powered solution, it's also environmentally friendly.
  
  Painless Installation
  The best installation feature is that the fan requires no electrical hook-up. That means no electrician and no complicated wiring is required, making the installation of your attic fan fast and easy.
  
  It mounts in the gable end of the home, behind a decorative shutter (not provided, see below). The solar panel comes with brackets that allow for mounting on the roof.
  
  Smooth Operation
  The solar panel collects energy throughout the day under direct sunlight and converts it into "free" power to operate the fan -- providing ventilation during daylight hours, when it's needed most.
  
  It creates up to 800 cubic feet of air movement per minute to remove uncomfortable heat in the summer and damaging moisture in the winter. And, the solar panel is wind-, hail- and impact-resistant to stand up to the elements.
  
  Specifications
  
  
  Moves up to 800 cubic feet of air per minute
  Solar panel collects and delivers power under direct sunlight during the hottest part of the day without electrical cost
  Wind-, hail- and impact-resistant solar panel
  High efficiency, durable 24-volt DC motor
  5-year limited warranty
  
  
  
  (source: airvent.com)  
  
  Model #SHT15W
  
  
  Plastic
  Sold in the color white only but it can be painted to match a home’s exterior
  Automatically opens when the power fan is running and then closes when the fan is not running