Espressif has just released their newest WiFi enabled MCU the ESP32-S2. It's a successor to the ESP8266 and has many improvements. Not only does it have more CPU, more memory, and more GPIO pins it's also cheaper. Just for posterity's sake here is the current cost for of WiFi enabled MCUs.
|ESP32 (gen 1)||$3.00|
|ESP32 (gen 3)||$1.50|
Note: This is the cost for the bare IC only.