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.