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.
Do you count conservative difficulty change +30% every 11 days ? This mean -30% mined coins every 11 days. And delivery time is important as well as you see from difficulty change