Walmart offers the Chamberlain Wireless Motion Alert System, model no. CWA2000, for $35.23. (Amazon charges the same.) Opt for in-store pickup to avoid the $5.97 shipping fee. That's tied with our mention from a week ago and the lowest price we could find now by $14. It features audio notification, 1/2-mile sensor range, weatherproof outdoor sensor with adjustable sensitivity control, and expandability up to four sensors.