Amazon offers Prime members the First Alert Smoke Alarm with Battery Backup, model no. SA9120BCN, for $7.78 with free shipping. (Non-members can spend over $35 for free shipping; otherwise, shipping adds $6.78.) That's the lowest total price we could find by $13. This 120-Volt hard-wired smoke alarm features a power indicator, 85-decibel alarm, battery backup, and silence/test button.

Update: The price has increased to $8.75.