Ending today, Beach Camera via Rakuten offers the Nest Protect Smoke and CO Alarm 3-Pack for $327. Coupon code "SAVE15" cuts that to $279.65. With free shipping, that's the best price we could find by $47, although we saw it for $13 less in December. They feature a 120º field of view to 20 feet and work with the Nest app to send motion alerts.
You don't need to hire a contractor or do extensive renovations to add value to your home.