300 BLK UMC is $10.36 per box of 20. That is cheaper than any 30-30, and 6.8 is 40% more expensive.
This is 300 BLK at 300 yards - 20 inches of penetration and full expansion from a 9 inch barrel!
So at 300 yards, you can expect it to make a 50 caliber hole completely through a deer even when using a 9 inch barrel.
Does 6.8 have longer range? Sure - especially if you use the 120 grain SST bullet. But it seems most 6.8 people love the Barnes 85 TSX, and that is 685 ft-lbs at 300 yards as loaded by Silver State Armory. Remington 125 grain 300 BLK ammunition is 690 ft-lbs at 300 yards, and the Barnes 300 BLK 110 grain in the photo above is 615 ft-lbs at 300. Just sayin' - will the deer feel the difference?
And 6.8 uses about 45% more powder - which ends up as noise/recoil/blast, more expensive reloading from the powder, and the brass is 2-3x the price. Also 30 caliber bullets are so common that they are actually cheaper than 22 caliber bullets in some cases (pulled bullets, for example).