Extremely cheap "Arduino" for single tasks? I have been thinking of a million things to do with my Arduino, but those things are extremely simple and it would leave my Arduino as a $25 waste. 

For example, programming Arduino to control my living room lights with my smartphone ... I was thinking of connecting the Arduino in the wall, onto the light switch wires. I am looking for a "permanent" solution, but I figure this would leave me with a $25 waste of circuitry inside my wall.

Are there any "Arduino" types of electronics that perhaps have many less ports, JUST enough to get a single task done?
Digispark comes to mind, or any of the ATTiny breakout boards at Tindie. Or, make your own PCBs, I just now finished soldering a few ATMega328p circuits to boards I designed. Microcontroller is $1.50, PCBs range from $2 to $8 each, time is "free."

Also consider "compatible with" designs; while you're not directly supporting the ecosystem, if you've already paid full price once and your conscience is clear, eBay has plenty.