Amazon has the First Alert Carbon Monoxide Detector (plug in with battery backup) for just $19.54 (reg. $25+). Installation is simple since this unit plugs into an outlet but it also has a battery backup. Expert recommend having a Carbon Monoxide Detector on each floor of your home and in each bedroom.
This is cheaper than Walmart, Home Depot and Lowes. However, you can get it for $19.59 at Target.
Shipping is free with Amazon Prime or a qualifying purchase of $25+.