What amazed me about that: Within 9 months, your profit drops below $1k/m. Barely three more months later and you're down to $250/m. So after a year, you're fast approaching the point at which the energy you put into the device costs more than the bitcoin money you get out of it.

Play with the knobs some more and you realize: If you purchase the 400 GH/s August device in December, you will never make back the 7.5k that the thing costs. Wait "only" until November and you barely get a 60% ROI.

That's some fast, fast market right there.