Holmes Twin Window Fan with Manual Thermostat – Link 
Holmes has this on sale for $39.99.
Better yet, once you add this to your cart, the total will drop to just $19.50 w/free shipping!
- Dual-blade operation
- Two speed-settings and manual-reverse air flow control
- Water-resistant motors are safe to use during rainy weather
- Designed to fit most double-hung and slider-windows
- Adjustable extender screen and extender panel help secure fan in larger windows
- 3-year limited warranty