Ending today, Beach Camera via Rakuten offers the Nest Protect Smoke and CO Alarm 3-Pack for $327. Coupon code "SAVE20" cuts that to $267. With free shipping, that's the best price we could find by $60. They feature a 120º field of view to 20 feet and work with the Nest app to send motion alerts.
There are lots of things Uncle Sam can't touch — so long as you play by the rules.