make your own calculation
in my eyes it still doable and profitable
I've spoken to a few. While 65nm could work you are probably better off teaming with someone like bitfury and buy a bulk of chips for cheaper. I mean that you could use a hundred thousand or more and bitfury's tech to have chips packaged at a high volume discount.
As I speak now I know 3 or 4 companies already doing 28nm.
The next generation chips will be 28nm or even 14-20nm. If you do come up with 65nm you are coming up a little late in the game. Especially being that there are already multicore ASIC chips out.