Cafago offers the Xiaomi Mijia Air Purifier Pro for $334.68. Coupon code "ZPAS0010US" cuts that to $273.83. With free shipping, that's $183 off list and the lowest price we could find. It's recommended for areas up to 60 square meters, and features an OLED display, smartphone app control, and temperature and humidity sensor.
Note: This item ships from mainland China and may take two to six weeks to arrive. Also, the countdown timer on the product page does not reflect an actual expiration time and resets daily.