The real OEM BG-E11 battery grip for the Canon 5D Mark III will run about $300 dollars. You can find that on the product page (click here). I'm sure the OEM is the best of the best, but i've managed to purchase several aftermarket grips for my T2i, 60D, 5D Mark II, and Canon 7D.
When I found the price dropping on the aftermarket BG-E11 grips, I decided to get one for the Mark III for around $50 dollars. You can find the grips via eBay (Click Here).
The Pixel images so far look very close to the original BG-E11 design including the small joystick button and rubberized coating (according to their description). The listing via eBay is posted as a Pre-order to be released June 10th. Check out more of the images and information following the link (click here).
Canon 5D Mark III (alternate) Aftermarket Battery Grip BG-E11