How to find a cheapest 3G GPS Tracker?

I think you will just need to use Google to find the cheapest. Good luck.

I'm using Rewire Security's 102 Nano gps tracker on my Phantom 3 Pro (hopefully on my phantom 4 soon .
They provide a cradle to mount the tracker on a phantom drone. and it's very easy to track it from my phone live, trough their live gps tracking platform: gpslive.co.uk

check it : https://www.rewiresecurity.co.uk/102-nano-gps-tracking-device-hard-wired-kit