The third-party Vello battery grip is now available for the Nikon D600 for only $79.95. Compare that to the $274 for the Nikon MB-D14 battery grip for the D600.
I previously tested the Vello grips for the Canon 5D Mark II and 7D, and found them to be solid performers – especially considering the price. Check out the Vello grip for the D600 here at B&H Photo.