The promoters of the .300 BLK are promising and showing ballistic tables with performance the same as the 7.62 x 39. AK ammo isn't highly loaded tactical stuff, it won't be that hard to push the performance with the right powder and very careful attention to leade and twist. If anything, the .300 Whisper crowd answered all the comparative questions years ago.
The advantage will be using the issue bolt and magazines. The disadvantage will be no military surplus, so regardless of how "inexpensive" is defined, I don't see less than $15 a box.
The two cheapest calibers out there, aside from .22, are 5.56 or 7.62x39. If blasting a lot of ammo for a low price is important, that the choice, no other viable option exists. If you want to shoot from the gun with the better set of operating controls, get an AR, if hunting and casual range use, get the AK. Getting one with the other caliber doesn't work because the designers made the mag wells incompatible. I say that having shot a Valmet in .223, nobody seems to think it was a good idea, I see no attempts to barrel an AK in 5.56. Hey, it's a cheap caliber, right, it should be a going deal! No, not so much. It's because nobody wants a varmint round in the AK. I don't see much talk about the 5.45 AK-74 on the boards, why is that?
More people want a big bullet in a small gun, apparently that seems to get their interest more. If they would make an AR lower that took the AK mag, the whole project would fly. Use a superbolt and go with it. Even Colt has built AR's to take the 7.62x39, they sold very few. It's all about the straight mag well, and if you CUT IT OFF, it might work.
As it is, you have a ten shot semi auto with .30-30 ballistics, and a Win 94 would be as easy to use.