I am also interested in this topic. some people say cost of mining is around 3 to 4k and some 7 to 8k. The reality is that since we have been at this prices last years, hashrate has nearly tripled. So at the same prices as 4 months before, miners are not as profitable, they have a lot more competition.
you have to consider a couple of factors:
1. difficulty (which everyone forgets)
2. bitcoin price
3. electricity and other costs.
2 and 3 are easy to measure since the numbers are simply found and 3 is a fixed number. but you have to measure how long does it take to earn a fixed amount, it can be finding of 1 block or earning 1
BTC or earning in fiat like $1000 or something like that. and this is constantly changing due to difficulty adjustments.
ps. I don't think cost of mining is important at all! because you may think it is $7000 now and say price won't fall below that but it may fall and then if it falls below $7000 the cost of mining will be reduced to $6000 and so on.