First Alert Store offers two First Alert Plug-In Carbon Monoxide Alarms with Battery Backup, model no. CO605, for $59.98. (Add one to cart and then update the quantity to two to see this price.) Coupon code "UMBRELLA1" cuts them to $29.99. With $5.95 for shipping, that's $17.97 per alarm and the lowest total price we could find for two units elsewhere by $10. It features an electrochemical sensor, mute button, and more. It requires two AA batteries.