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.
As the other poster mentioned, they've typically been $35. The barrier of entry for IoT is still pretty low though, you can get an ESP8266 for ~$2/ea. That'll handle wifi and can also be your application processor. For another ~$2-3 you could add on a BLE module too.
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)