The HC-05, a Bluetooth to serial bridge, can be found for around $5 on the internet and therefore may be the cheapest way to add Bluetooth connectivity to your project. However, its default setting… ...

“Which came first, the HC-05 serial-flash-hack, or the wireless Bluetooth Gamepad? ” Our guess is that [mitxela] wanted to play around with the dirt-cheap Bluetooth modules, and that building ...

The $22 kit includes an Arduino-BLE board and 6-pin header that should be fairly broadly applicable for building Arduino-based hardware that can communicate with iOS devices.