Etekcity via Amazon offers the Etekcity Standalone Smoke Detector & Fire Alarm
, model no. 679113376847, for $17.98. Coupon code "DETECTOR" cuts it to $11.98
. Plus, Prime members bag free shipping
. (Non-members can spend $35 or more to bag free shipping; otherwise, shipping adds $6.94.) That's $16 off list and the lowest total price we could find. It features a built-in Lithium-ion battery that lasts up to 10 years, 85-dB alarm, and test button that can silence false alarms.