Amazon offers Prime members the Optimus 1.5-Gallon Cool Mist Ultrasonic Humidifier, model no. U-31002, for $22.98 with free shipping. (Non-members can spend $35 or more to qualify for free shipping; otherwise, shipping costs $9.26.) That's the lowest total price we could find by $23. It features a water empty automatic shut-off switch, refill light, and adjustable mist volume control.