Jarden Consumer Solutions offers the Holmes Personal Ultrasonic Cool Mist Humidifier, model no. HM500TG1-UM, for $34.99. Coupon code "L1HV69A" cuts it to $12.90. With free shipping, that's the lowest total price we could find by $16. It features a 1-gallon tank and up to 24 hours of use per tank.
Note: It's not guaranteed for delivery by Christmas.