“UPB communication is a method of reliably communicating command, control, and status information across an electrical AC powerline. Because of its low cost and high reliability characteristics, the UPB communication method is ideally suited for command and control applications in both the residential and commercial markets. “ – For further details please refer to UPB Description.
One of its disadvantages it has only widely spreaded in the US (120VAC 60Hz). This article focuses on the European (230VAC 50Hz) electrical network, but with a little firmware modification it is possible to adapt it to the US mains. The price of an UPB device is around $80-$100, with this article you can build it from ~$8.
Cloning the UPB home automation system - [Link]