ipschoser1,
It is perfectly legal to take an existing pre-ban magazine and replace parts, including the mag-body, as long as you do not manufacture a "new" magazine, and the mag works in the gun it was designed for.
This is exactly how you get an AR10 mag from an M1A mag: you buy an M1A mag for $40-50, and then replace the mag body and follower with the $30 kit from Armalite. Yes, that is expensive in the M1A/AR10 case. This is also how IPSC shooters replace their high-capacity STI/SVI magazine bodies when they get stepped on.
So to produce a 6.8-compatible mag with these alleged new mag bodies, the cost would be $15 or whatever for the original pre-ban AR15 magazine, and then the cost of the replacement mag body.
They won't be as cheap as normal AR15 magazines, but they won't be priced out of reach. Remember, a person who wants 6.8-compatible magazines will already be willing to shell out a premium to either buy or reload 6.8 ammo.
-z