At 100 or 200 billion difficulty, the price of bitcoin would probably have to be a few thousand dollars to cover electricity costs. That's why it can't go that high. Even Cointerra miners aren't that efficient.
Using cointerra's number, at 100B difficulty, using todays BTC exchange rate, maximum electricity price for operational profitability is ~$0.06/KWH
http://mining.thegenesisblock.com/a/ffa2bc17c6I admit remembering wrong, and I thought I used 0.15, but that price is still within reach of many industrial miners.