Monoprice offers its Monoprice Z-Wave Door and Window Sensor, model no. 10795, for $19.99 with free shipping. That's tied with our March mention as the lowest total price we've seen. (It's the best deal now by $11.) It includes mounting screws, adhesive tape, and a CR123A battery for installation into an existing Z-Wave network.
Also available is the Monoprice Z-Wave Motion Detector with Temperature Sensor, model no. 10796, for $22.99 with free shipping. That's $7 off list and the lowest total price we could find, although we saw it for a buck less in March. It features an infrared sensor able to detect movement up to 100 feet away.