[Snorlack]

It's already $4 with the new ESP8266-EX from WeMos!

Full WiFi stack, 11x GPIO, Analog in, SPI, I2C, 80/160Mhz, 4MB Flash, power/flashing over USB, supported by the Arduino IDE, or just use the NodeMCU Lua firmware with modules for most sensors, displays, MQTT etc. I'm having a blast with them right now.

(Edit: http://www.wemos.cc/wiki/doku.php?id=en:d1_mini)

[mciancia]

WTF is WeMos? ESP8266 module itself costs below $2

[Relys]

Ummm, I believe it's a board you can surface mount the ESP8266 on to.