[joezydeco]

But I like jbob's idea.

You can cut a huge chunk of the cost out of the Nest if you drop the LCD, touchscreen glass, and fancy stainless steel housing.

It could be a simple plastic box with the CPU, radio, sensor and control relays inside, and then a hidden mechanical thermostat knob/control to use as a floor + emergency override if the internet cuts out or the phone goes away.

[jbob2000]

That's what I was getting at! (in an admittedly crass way). Split the device between temperature sensing and temperature control.

[joezydeco]

That said, there are devices that come extremely close to this, like the RadioThermostat CT-50. It has it's own HTTP API so you technically could operate it on your own.