Kidde Hardwire With Battery Backup Photoelectric Smoke Alarm – Link 
Amazon has currently dropped the price on this to only $7.57!
This normally sells for $18-$20 and is now the lowest it has ever been on Amazon.
Note that this is an Add-on Item which ships with a $25 Amazon order.
- Photoelectric sensing technology
- One button design, operates hush and test
- Large mounting base, 9V battery backup included
- Tamper resist feature
- Pre-stripped wiring harness with tinned strands