I'm sure Zeikos will come out with a $120 grip before too long, and then a bunch of chinese knockoffs will appear for $60 ea. But, you are complaining about the price of the grip for a $3500 body? Seriously?
And to be honest, Grip is just that... a grip. An accessory that lets you have more battery life, extra controls for vertical shooting and better weight and ergonomics when you use heavier lenses.
Why spend so much on it?
No, you are wrong! It is not just a grip. The life of the camera is hanging on this item. Take a case of a wedding photographer who has 2 cameras suspended to the BlackRapid FastenR-3 hardware. If the tripod socket is not able to carry the combined weight, then you can kiss your 5D3 and your lens goodbye!