The $1,200 I quoted came from a CASA witness at the Beaver coronial inquiry, and was in AUD including installation. But that's just for a panel mounted CO detector without any ADS functionality. That’s an awfully expensive CO detector even taking exchange rates into consideration.
A highly accurate, 'pocket' CO detector with aural and visual alerts and Bluetooth capability costs around AUD300. That’s AUD2.5 million rather than AUD10,000,000 for the entire piston engine fleet in Australia (based on the fleet size used in CASA’s calculation). And if the mandate applied only to aircraft carrying fare-paying passengers, that brings the overall 'fleet cost' down even further. It appears to me that CASA does not put a very high price on a fare paying passenger’s life.
Your mention of equipment, like Sentry, with ADS-B IN functionality and traffic alert functionality and accurate CO detection and alert functionality points up another strange aspect of the ATSB’s current focus. Out of the Jerusalem Bay Beaver tragedy, ATSB recommended that CO detectors be mandated! That was rejected by CASA. I would have thought that if ATSB considers ADS-B IN and CO detectors to be such great enhancements to safety, ATSB would be extolling the virtues of the available ADS-B IN systems that include an accurate CO detector. (Does the Sentry WAAS GPS work here?)
(Those chemical CO cards change colour just after the pilot is sufficiently poisoned not to notice.)