Can anyone recommend a relatively low-cost fitness tracker type of device that I can interface to with my own code (eg via CoreBluetooth on iOS)? Most of those that I've come across are seemingly expensive ( > A$200)

Can anyone recommend a relatively low-cost fitness tracker type of device that I can interface to with my own code (eg via CoreBluetooth on iOS)? Most of those that I've come across are seemingly expensive ( > A$200)

The device should at least have movement tracking capability. Other metrics are probably a bonus