Amazon offers Prime members the Leeo Smart Alert Smoke / Carbon Monoxide Remote Alarm Monitor for iOS and Android, model no. LNL9ZA1CB, for $29.97 with free shipping. (Non-members can spend $35 or more to qualify for free shipping; otherwise, shipping costs $6.74.) That's the lowest total price we could find now by $20, although we saw it for $2 less in January. It monitors your existing smoke and carbon monoxide alarms and can be configured to send alerts via smartphone and automatically notify emergency services.