$1.50/GH
He never published it. It's an estimate based on FC's old balance sheet, where he had listed the total cost of making all the hardware. Then I figured about how many TH he'd produced based on his total sale profits divided by price/TH, plus the known farm size (~50 TH).
Not only are you basing your guesstimate on 3 of your own guesses, one of those three happens to be derived from the number you are trying to calculate! Awesome logic.