You assumed prices of $10 per Ghash/s. It is well-known current ASIC vendors have insane margins on the raw silicon. As a matter of fact, Cointerra is already selling pre-orders $3 per Ghash/s. Watch the price drop to $1 or less per Ghash/s over the next 12 months. Therefore a $100 million investment in mining hardware is all you need for the network to grow to 100 Phash/s. This will happen by September 2014. Watch.