it all really depends on what the Bluetooth device is. If it's headphones or a game controller or something like that, it wouldn't affect your data usage. OTOH, if it's something like connecting your phone to your computer using Bluetooth, it certainly could, but it would be the phone using the data, not the actual Bluetooth. Bluetooth is just a method of connection, like a LAN cable or speaker wires.
What type of Bluetooth device are you going to be using?