$5 for a Raspberry Pi Zero! 
https://www.raspberrypi.org/blog/raspberry-pi-zero/
More at the link.
It's pretty hard to drop the price much lower than that. I'm thinking of getting a couple just to see what they're like.
However, they're out of stock:
http://www.adafruit.com/products/2885


https://www.raspberrypi.org/blog/raspberry-pi-zero/
Quote
Of all the things we do at Raspberry Pi, driving down the cost of computer hardware remains one of the most important. Even in the developed world, a programmable computer is a luxury item for a lot of people, and every extra dollar that we ask someone to spend decreases the chance that they’ll choose to get involved.
More at the link.
It's pretty hard to drop the price much lower than that. I'm thinking of getting a couple just to see what they're like.
However, they're out of stock:
http://www.adafruit.com/products/2885



















, and when adding converter-cables and a power supply it passes €25,-
maybe it's the name that says it all...