Buying the components directly from the producers, the hardware is not so expansive (but it take a lot time to wait ). Let's say about 300 USD.
The very challenge part is designing some 3D parts and having the patience to test them, re-printing until they fit, etc. finding the best wire connections, soldering Y cables and modifying the original RC chassis. A lot of hours of work!
Jason, I'd like it but I should have almost a radioisotope thermoelectric battery in the bird!
Jokes apart, having so much power reserve in the radio controller give me a great margin of safety. Every thing could still happen, but unlikely a loss of signal.