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).
Aftermarket BG-E11 Canon 5D Mark III Battery Grip