The Simple Wi-Fi HC-21 module is a meager $10-$15 bucks on ebay right now. It is a simple Wi-Fi serial port server or virtual serial port. This means it is self contained and will handle all the overhead of negotiating a Wi-Fi connection and communication leaving more room for your application in the Arduino.
X10 Lighting and appliance modules are cheap on Ebay, I wanted to control them with my HMI (ScadaBR) so I put a X10 and Modbus library together in an Arduino to turn on and off the modules.
A Programmable Logic Controller, PLC or Programmable Controller is a digital computer used for automation of electromechanical processes, such as control of machinery on factory assembly lines, amusement rides, or light fixtures. PLCs are used in many industries and machines. http://en.wikipedia.org/wiki/Programmable_logic_controller
A HMI (Human Machine Interface) is the system by which people can interact with a machine. The user interface includes hardware (physical) and software (logical) components.