Walmart offers the First Alert Battery-Powered Carbon Monoxide Detector, model no. CO400, for $14.11, excluding the mentions below. Opt for in-store pickup to avoid the $4.97 shipping charge. That's the lowest price we could find by $4. It features a silence button, front access battery compartment, and end of life alert.

Close prices: Amazon offers it for $14.59 with free shipping for Prime members. Target charges the same with free in-store pickup.