You need to factor in the cost of HW as it will be useless after 4 months.
A more reasonable estimate on production cost per btc is 600.
Not necessarily. Do you think that the large farms of Bitfury haven't already recovered their initial HW costs? How about KnC? Or maybe Bitmain?
Further, to derive that $600 figure do you know what the cost per GH is for these large corporate farms? If you do, could you post the citation for said figures?