Its correct that the advantage of lower process nodes arent so high in cost like it looks on the first view since a chip is way more expensive. But a big factor to keep in mind is that you save a lot money by assembly, you need less miner boards, you need less work to do for the same TH and you need less space. Not to forget the higher speed to bring up TH.
So as long as you dont plan to sell... for mining lower process nodes have some good advantages.
Yes, I think it should be the main factor in the move to 40nm. As I said in my previous post, moving to 40nm is probably cheaper than scaling the 130nm technology to 1PH (hardware costs aside, you would need more datacenters and pay more in electricity - that quickly surpasses 200K$).
40nm would also be good for USB miners. The current crop is very much useless but AM still sold boatloads of them.