I myself have ordered a large machine with a friend (less than 1Th/S but not small) and estimates show it should "pay itself back" within 30 days. The machine is $6.50 USD per Gh/S. Even if BTC falls to $100 and we assume a ridiculously high profitability decline of 0.1 or even 0.04 (unrealistic by the worst math), the machine pays itself back and turns a profit in a 90-day period. However, we are not looking to recoup USD, as we are looking for a set amount of BTC to break "even." Much of this BTC will go into EMA trading and Gh/S rental for continuing compounded income, hashing speed, and holdings.