It seems that its bottom is around 3600$ , and I say that because of the miners , mining 1 bitcoin needs a lot of power and I think it coasts around 3600$ , and if it goes deeper than that it wouldn't be profitable to mine , Sometimes its diagram acts ridiculous , dropping 12% in one day means some people still are playing with it .
It is not possible to say at which price tag mining becomes unprofitable. As price goes down, some miners exit the proces and others receive bigger share of the block reward, therefore become more profitable.
So determining bottom by the cost it takes to mine 1 btc is not possible. If majority of miners stops mining today, price for mining 1 btc might drop to 10$...
And there you have also miners who are mining on renewable energy sources, and other low energy cost solutions, which drives expenses lower. So what will this bear market do : Well, it will force miners which are not optimised from a prospective of costs/performance out of the proces, and only the strongest will survive, which is kind of a natural selection...
So for the most advanced miners there is no such thing as nonprofitable mining, because more the price drops more other miners quit mining, and bigger their share of reward gets... and this point where other miners start leaving the proces is way above their breaking point...