The cost of mining does not determine the price of the coins at any time, they are probably not correlated at all ... but it can act as a floor (elastic lower bound) to the price during down swings. This is an important dynamic to keep in mind, so doing the kind of calcs as in the OP are useful (and worthwhile), although doing them properly is becoming ever more complex with hardware evolution (ASICs), decreasing supply/total btc ratio amongst other factors.