OEM Panasonic Battery Grip will run you $199 dollars (as seen here). The good news is now there are aftermarket battery grips available for the Panasonic GH3 camera starting around $65 bucks. The bad news is that they still have the tripod mount located offset from the optical center, which mainly sucks if you're trying to use a set of rails.
Still, I guess you can't complain when it's $135 dollars less than the OEM version. Some listings include an extra set of batteries and charger. Check them out already available on eBay (click here).
Aftermarket Battery Grip for Panasonic GH3 Battery