Tthere is a special offer on SONOFF module on Banggood from today until 10th of January. You can buy the module for under $6, $5.99 ( 5.79 if you order it from mobile application).
The SONOFF module is produced by Itead.cc and can be used with their mobile app eWeLink or you can write your own software.
The core of the module is the ESP8266 and is connected to a 5V relay capable of 10A ( I will not bet on that)
I already ordered few of them and I am planning to put my own software so in the end I can :
-on/off the relay from my mobile app
-give a vocal command to trigger it on/off with Amazons echo dot 2 help
-set cron task for relay.
The promotion will end in January so if you are reading this post after this date it may be outdated ,but you can check the Itead.cc site.