It's not that they'd stop making them when ROI hits the 3c/kwh electric level - it's that folks stop buying them when they can't make any money on them.
Guys, while a datacentre full of mining equipment will draw a lot of power, it's nothing compared to an Aluminum smelter, Steel Mill, Iron Foundry etc.
Even a large manufacturing plant such as a car plant would likely draw more and there are plenty of those in China.
Generation cost varies across regions but if you do your homework you'll find that the raw cost of electricity generation in China is around the mythical 3c per kilowatt (it varies per region, per technology: gas, coal, hydro, nuclear).
The market rate
after the government subsidy of 0.42 Yuan/kWh seems to be about 0.66 Yuan/kWh (10c) but there are big discounts for larger users and direct purchase schemes for the really big users such as those above. Those very largest industrial consumers in the list above are paying about 4c per Kwh, so I would imagine Bitmain are paying something like around 6 -7c per kwh (in the middle of the range).
If you think how politics influences subsidies, subsidies go to the largest employers so I don't think Bitmain are going to qualify for anything special there.
We'll never know the exact number, but it's not 3c.
Like everyone here, I'm trying to work out how this will play out and good data beats FUD every time.