I'm not good at embedded stuff, but I imagine it would be something along the lines of 200 hours of work for someone competent, at, say, $100 an hour. I would hope that they're selling at least 1000 of them. That's $20 per unit for software costs.

Again, this is without the overcomplicated UI on the front. My opinion is that if you're selling to an admittedly niche market that's looking for a network controllable thermostat, there's no need to have any advanced UI on the unit itself. Maybe a simple override controller, but that's it.