Globalprice via Amazon offers the Trademark Global Super Door Stop Alarm, model no. 72-0850, for $3.75 with free shipping. That's the lowest total price we could find by $7. It prevents the door from opening and activates a 120 dB alarm. A 9-volt battery is required (not included).

Note that orders placed today have an estimated delivery date of December 22 to December 24.

Update: Globalprice via Amazon has sold out; however, CZ-Two via Amazon offers it for $3.75 with free shipping.