I think making changes to the silicon it probably the most expensive. Every new design needs a process of testing , trialing and you do loose quite a few bucks doing so , hence the companies like tsmc will charge a lot for new designs. The litography is very expensive , the yields for new designs are quite poor as well.
I was reading somewhere that tsmc charges nvidia around 30k per wafer. That is a lot. With new designs and lower nm processes , the cost has skyrocketed. It s becoming very dificult to make new chips at that level.
Needless to say , that in this case , the hardware has nothing to do with it. It s a pure commercial decision from AMD. nothing more.