Update: For Mark II owners, Amazon has Zeikos ZE-CBG5DII Multi-Power Battery Pack for $50 with free shipping. Thanks, Andy!
Sold out.BuyDig.com has the Zeikos Vertical Battery Grip for Canon EOS 5D Mark III for $50 with free shipping. Functions as both a high-capacity battery pack and a vertical grip. The Canon-branded grip goes for at least $275.
Amazon has it at the same price http://www.amazon.com/Zeikos-ZE-CBG5DII-Multi-Power-Battery-Canon/dp/B00246XB1S