7.62x39 is by far the cheapest, and the CZ-858 is the highest quality firearm you can buy that will deliver this round.
.223 is also very cheap, but most .223 rifles are restricted.
After that, I dunno...
You can still get surplus 7.62 Nato for about $.55 a round, if you look hard enough.
7.62 x 39. Can get 1120 rds for $195 to $210.
but most .223 rifles are restricted.
wrong, there are tons of .223 rifles which are non-restricted.
Since when are most .223's restricted?