The Sensi Smart Thermostat is currently on sale over at Amazon for just $99. Making it the cheapest "smart" thermostat currently available. The Sensi may not look as pretty as the ecobee4 or the Nest Learning Thermostat, but it is also a fraction of the price.
Sensi's Smart Thermostat does work with Amazon's Alexa. So you are able to control it using your voice, without even touching it. Of course, there is an app available as well, for controlling it from your smartphone. It's also super easy to install, Sensi says it should take less than 15 minutes to install it yourself. Of course, you could have a professional take care of that, but that really isn't needed. It does have geofencing available, so once it knows you're home, it can turn up the air conditioning or the heat (depending on the time of year) so that you're comfortable, but also save energy when you aren't home.
It's also eligible for Amazon Prime's free shipping. If you are not a member of Amazon Prime, you can sign up for a free 30-day trial and take advantage of unlimited two-day shipping as well as the many other perks that Amazon Prime includes.