dpends on how optimistic you are regarding price and network difficulty growth slowing down over the next 12 months. At $0.13 USD/kwh my assumptions include >$1000 by 2016 and difficulty growth sticking around 4-5%/jump for the next while, at least until price growth. Theres also the fact that hardware often sells used for 2-3x its mining value near the end of its life. Right now, S3 units are still selling for $200 and the S1 goes for ~$50 on sites like kijiji, craigslist, ebay, despite the fact you can get new hardware that is similar hashrates but more efficient for similar $/GH.