Care to say which of those 3 is derived from the number being calculated? It is most certainly not clear. Are you confusing sale price and manufacturing cost?
Sale profits.
Using the sale price of the hardware (which was known), it is possible to determine how many blades were manufactured for sale based on the sale proceeds. Hardware deployed is known: 50 TH (FC did disclose this number, look through his post history). Summing these two and dividing by manufacturing cost gets you a cost per hash figure.
I have no interest in finding the scratch paper where I did this nor in typing it up. Do it yourself.
As mentioned, USBs are a confounding variable and require some guesswork. If their manufacturing cost was significant, then $1.50 is too
low an estimate, which I am obviously comfortable with.