Amazon offers its Prime members the Chamberlain Wireless Motion Alert System, model no. CWA2000, for $29.37 with free shipping. (Non-members can spend $49 or more to qualify for free shipping; otherwise, it costs an extra $6.94.) That's $6 under our mention from a week ago and the lowest price we've seen. (It's the best deal now by $6.) It features audio notification, 1/2-mile sensor range, weatherproof outdoor sensor with adjustable sensitivity control, and expandability up to four sensors.