I do not think the cost of a parts pack is the same as a kit for a few reasons. Before I go on I will say that we have been taught that was the truth about everything for decades.
I am going to speak of a newly tooled engine parts pack.
1) CAD because the files can be moved and transferred over from any already designed engine and parts.
2) They can cut more than 1 cavity into the mold because the molding machine platen size will allow it. Maybe even 4 to 6 cavity's So every time the mold closes they have 4 to 6 parts packs to sell not just 1. I would not be surprised if they could have an 8 cavity mold for engines.
Now I will say that the packaging is where the costs have not changed much, they still have to stuff them in the box and wrap them. Yes it is faster to put the parts pack in the boxes, and they can also put more in the case to ship back, that part of the overhead is what it is.
I agree with Luc on the 3D part in new tools.