I'm not so sure that it is a cost reduction. It doesn't cost very much money to buy a shaft and some stamped rocker arms. The machining on the head might be a tad more expensive for the shaft since it requires a curved surface, but at the volumes these parts are made I doubt there is much difference. That curved portion is probably made with a large custom mill or broach and I bet it only takes a few moments.

I think the original shaft design is less expensive than the AMC design but without having the quotes in front of me I couldn't say for sure.

The Magnum engine has a bunch of items that cost more than the LA design though so it wouldn't surprise me that they added money in the valve train also. I don't think the Magnum engine was designed as a cost reduction. It seems to me that they were more concerned with warranty reduction and improved service life.

Hyd roller lifters are more expensive, the front belt drive is more expensive, 10 bolt valve covers are more expensive, etc. There are a lot of cost increases in the Magnum engine if you look at it closely and I think the valve train was also a cost increase. I'm not sure why they changed the design, might have just been politics. Some guy who liked that design was put in charge so he changed the design. That stuff happens all the time.