it should be noted that for that to make any sense the quantity X needs to be a function of time, which it is not.
Sure it is. Total hashing rate changes with time.
And here's a graph of it.So, what does the price / profitability need to be at 7M difficulty, which it will hit in 6 more increases, given 50% difficulty-jumps per increase?
Well that depends on mining hardware and electricity costs. But according to this it would still yield a profit even at 17$ a bitcoin! Try it!
