For in-store pickup only, Best Buy via eBay offers the PEQ 3-Series Contact Sensor
, model no. 3300-P, for $19.99
. (Add it to your cart to see this price.) That's tied with our August mention as the lowest price we've seen. (It's a current low by $12.) Designed for use with most ZigBee HA 1.2-compatible platforms, it detects the opening and closing of doors and windows and includes all mounting hardware.