Monoprice offers its Monoprice Z-Wave Motion Detector (pictured), model no. 10796, for $24.18. Pad your order over $25 (this Null Modem Adaptor costs $1.02) and apply coupon code "5FEB" to drop the price to $19.18, excluding padding. With around $3 for shipping, that's $5 off and the lowest total price we could find. It's compatible with existing Z-Wave networks and features infrared and temperature sensors.
Also available is the Monoprice Z-Wave Door and Window Sensor, model no. 10795, for $24.88 before padding your order over $25 and applying the above coupon, $19.88 after (excluding padding). With around $3 for shipping, that's the lowest total price we could find for a similar sensor by $11.