With great pride for my colleagues, known and un [personally] known, I can say that I have no idea where the cheapest flight training can be found. It certainly must be somewhere, but I have no idea where, and am not looking. Similarly, a pilot presenting themselves to fly my airplane, who states that they sought out the cheapest flight training they could find, will be viewed with caution to fly my planes.
If you want to economize on your flight training, choose a school which is practical for you to attend frequent training, and go to each training session very well prepared! That's the best bang for your buck!