Home Depot offers the Lasko 8" Electrically-Reversible Twin Window Fan, model no. 2138, for $49.99 with free shipping. (Amazon charges the same.) That's the lowest total price we could find by $2, although most retailers charge at least $55. It features an adjustable thermostat, automatic control, fused safety plug, and independent motor control. It fits windows from 25" to 35" wide.