Amazon offers Prime members the Black & Decker Table Top Ultrasonic Humidifier, model no. BXHU090, for $11.93 with free shipping. (Non-members can spend $35 or more to qualify for free shipping; otherwise, shipping costs $7.61.) That's the lowest total price we could find by $27 and one of the cheapest ultrasonic models we've seen. This cool mist humidifier operates up to 10 hours on one tank.