I am not sure how much 6.8 ammo is I've never looked into it, but if you think it is pricey... The cost of the ammo is going to dictate which direction you should take. 300 is cheap, but not that cheap. 7.62 x 39 is cheap altogether.
If you reload, then you can cut the price of your ammo and you have a wider choice. I got into reloading way back when I was shooting a lot of 10mm. It has come in handy for all my pistol rounds. It drops the price of my Grendel rounds somewhat, but I like reloading for it as I have more control.
If you are going by ammo cost, then the 7.62x39 is probably your best bet.