Price sort of depends on features, what the market will bear, etc. A cradle accessory that also dispenses with the need for a $30 or more dollar protective bag means the cradle offers that value, too.
A battery cradle could be designed around some commonly used battery like the Ravpower Element, one of the Ankers, --some known reliable battery from a vendor that hopefully will keep making them a good while. I think this is where Sound Devices got it right when they picked the Sony L type battery for the 7xx series--long run times, reliability and long production runs. Some resourcefulness in picking the right battery would pay off.
I've always mounted my camera on top of my 60d and the 70d, and I don't knock them around any. But mostly I use the 60d for video stuff.