Not the cheapest it's ever been but a good price nonetheless for someone looking for these. I recently had to replace both of my smoke alarms and bought 2x of these from TGG ebay for $151 each so I'm kinda spewing now but what can you do.
Good Guys $139:
Wired:
https://www.thegoodguys.com.au/google-nest-protect-smoke-ala…
Battery:
https://www.thegoodguys.com.au/google-nest-protect-smoke-ala…
Google Nest Protect Smoke Alarm (Wired)
Model:
S3003LWAU
SKU:
664455
PLU: 813917020814
Mains Connected
240 V Connector
Three long-life AA
Energizer® Ultimate
Lithium backup batteries
Voice alarms with custom location
Detect Carbon Monoxide
Nest app Mobile Alerts
It's funny how they removed the APIs for these things and then never provided a replacement (last I rememebr they aren't even in Google app)