Upload & Sell: On
The genuine Nikon grips are more expensive for a number of reasons - the newer ones of late have been made from magnesium alloy, the same as the camera bodies. Why would you put a flimsy plastic grip on a rugged pro/semi-pro camera body? Nikon also had to design them from scratch, which means lots of trial and error, prototyping and testing - incurring costs that have to be recovered from the final product. Not every camera buyer wants a grip, so there are much fewer products to off-set the design, tooling and set-up costs.
The knock-off copy manufacturers, buy a Nikon grip and reverse-engineer it. The really hard work has already been done by then. And then we have the relative costs of die-casting magnesium alloy vs moulding plastic.
In a nutshell, you get you what you pay for. I'm sticking with Nikon.....