Extremely cheap "Arduino" for single tasks?
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.