Amazon offers Prime members the Chamberlain Wireless Motion Alert System, model no. CWA2000, for $39.09 with free shipping. (Non-members can spend $49 or more to qualify for free shipping; otherwise, shipping costs $7.03.) That's the lowest total price we could find by a buck. It features audio notification, 1/2-mile sensor range, weatherproof outdoor sensor with adjustable sensitivity control, and expandability up to four sensors.