Re: the $500/BTC, we're all hoping but it seems rather sketchy to base the numbers on that since you can't order an SP35 using $500/BTC without a time machine.
That was actually a mistake, the fixed BTC price was only meant to apply to mining income. The initial purchase cost is meant to use the actual current BTC value. I'll fix that now, but I'll keep the $1800 for PSUs until more info becomes available.
I'll fix that now.