That's not correct. The SP35/31 consumes 3600-3800W to achieve 5.5TH. Rerun your numbers with that and you'll see why he made the offer he did.
Must have misread the specs, i went over them pretty quickly... Anyways, with a much higher power consumption, the break-even time will be long after the halving, and will indeed make a 0.5BTC bid more likely.
However, i still remain at my conclusion: everybody is free to make a bid, the OP is free to deny any bid
