Walmart offers the First Alert Battery-Powered Carbon Monoxide Detector, model no. CO400, for $14.19. Opt for in-store pickup to avoid the $4.97 shipping charge. That's tied with our February mention and the lowest price we could find by $3, outside of the mentions below. It features a silence button, front access battery compartment, and end of life alert.
Of note, Amazon offers it for the same with free shipping for Prime members. Target also charges the same with free in-store pickup.