Great thread btw...
We are working on a system similar to this. Using a flow senor like "Cahawkins51" mentioned in their post the Hall Effect sensor would be cost effective, cheaper then a water meter. (even with our dealer prices water meters ($45 range)) The downside of using the flow senors is the accuracy of the GPM with under 1 GPM leak... where as a "water meter" will still capture this reading.
Currently we use our wireless transmitters to capture the pulses from the meters for cities and multi-family markets, and send this info to our cloud and then accessible through our app.
We have prototypes in place now, all we need is to be able to integrate our data (gallons used, high / low alarms, battery levels and more into the Rachio interface.
** our transmitters will wake up every 59 minutes and report in... so we know the status of each, or when ever 10 or more gallons of water is being used. So we capture usage in real-time. this is great for real-time leak notifications. Battery life is 8yrs with our standard battery.