The entire rotating assembly does NOT need to be "rebalanced", only the crank itself. When they advertise "internally balanced", they don't mean it's already balanced, they mean it's suited to the internal versus the external. Since this is a 400, it IS externally balanced "stock". If you get an "internal" crank, you'll need a new balancer and flywheel/flexplate.
The imported cast cranks have shown to be a lot of trouble. The only "cast" crank we use for a 383/400 is the genuine Chevrolet. That nodualr iron crank is pretty good. The forgings from Eagle, etc. are quite good.
A balance "job" is $200 here (Richmond area). When converting to "internal", it can get expensive due to the cost of the "heavy metal" (tungsten) and the labor to install it. I've never heard of varying "degrees" of balancing costs, based on "how far". All the shops I know balance to within 1 gm. SAE standards are 1/10th of 1% of the entire rotating mass. In the case of the SBC, that's about 25 gm.