LightInTheBox offers the Napok H6 Wireless Intercom Doorbell in Gold or Silver for $16.99. Coupon code "DB5544680" cuts it to $15.99. With free shipping, that's $25 off list and the lowest price we could find. It features an adjustable speaker volume, outdoor and indoor bells, and 980-foot range. Four AAA batteries are required, but not included.
Note: This item ships from China and may take two to five weeks to arrive. Uncheck "Add Shipping Insurance to your order" during checkout to remove the 49-cent fee.