I can control a relay from an Android smartphone using Arduino and Bluetooth as seen here.
However, it seems too costly to be using Arduino and a Bluetooth receiver for driving a switch. As long as Bluetooth is a radio frequency, is it possible to make a simple Bluetooth receiver which can output 1 or 0 to drive a relay? If yes, how tough that is going to be?
The main factor here is the cost, which should be \$1-$5.