a 28nm NRE is several million, plus the first wafer orders (and possibly the second order too if you want more in a reasonable time) plus the first batch of all the parts to make the box.. so you're talking minimum $5m to start production!
$5M is at the high end of my estimates, keep in mind a bitcoin asic in all likelihood is much simpler than typical soc's/gpu's/etc and requires only a minimum number of layers/masks, but generally I agree and you are in the right ballpark. But so what? We were talking about marginal production cost, NRE by definition isnt part of that. NRE is a sunk cost, and therefore doesnt matter to pricing during the 'end game", when miners margins have dropped so low that vendors will have a choice between not selling anything, or selling something above marginal cost. If there is no longer a reasonable operational profit margin to be had, vendors may call it quits, but as long there is a profit margin to be had, they will sell (or self mine), NRE be damned. IOW NRE will be a key factor determining if vendors end up making a profit or not (and IM pretty sure they all will), but its not a factor in pricing.