This deal has ended.
Amazon has a 3-pack of First Alert Smoke Detectors with battery included for only $20.49. The 3-pack retails for $45 and you can't find a single detector for less than $10. These have a silence/test button, blinking power light and 85-decibel alarm. Shipping is free if you spend $25 or are an Amazon Prime member. There is no tax (except KS, ND, WA).