Demand drives up the price tag. Of course, as with mine, price is negotiable.
The GDE tune Turbo Brake option is what most of today's trucks use for engine braking (with the exception, you can get a real exhaust brake (loadleash) for 6.7 Cummins made by PacBrake). They basically adjust the turbo vanes for maximum back pressure. It works very well, IMHO for a 3.0 liters.
GDE does not offer a tuner, nor does any company that tunes the ECM, as no one has cracked the ECM through the OBDII port. They simply reprogram the origional ECM. They are coming out with more options, like real oil pressure monitoring (Can you believe that a lot of vehicles these days have fake oil pressure readings?? Google it).
As far as tuning a new truck, I did it on my first tank of fuel. What GDE does to the truck is amazing.