Figure about $20 in parts if you count the speaker, $25 if you don’t want to build the amplifier yourself. I like this build since it re-uses parts, and POWERS the bluetooth handset (most other builds require you to recharge the thing). I wonder how long before folks start adapting NFC transmitters for this… http://www.instructables.com/id/Make-your-own-Low-Budget-Bluetooth-Music-System/