1 TH @ your price: 3500$ all inclusive.
1 TH will mine ~5.09 BTC in a year so far with current increases in difficulty.
5.09 BTC is a lot less than 3500$ so investment doesn't even ROI until BTC/USD goes to at least 700
Regards.
Everyone has their own projects on difficulty, unfortunately most are wrong. A ~50% increase in difficulty/month for a whole year isn't sustainable which is what your calculations tell me you are using.
You are basically saying difficulty will go up to 151 Billion by December which is 17x the Hashrate currently. ~1200PH or 1.2EH. This will never happen at current pricing.
We're going to see a steep decline in hashpower going online percentage wise if BTC stays at the same value. If BTC goes up then equipment retains it's value longer and you can cash out to USD for more but you will also see more miners come online.
My Calculations show at least 9 BTC being earned by a TH today within a year, however I do love when people use the genesis/trade block for their calculations keeps more people out of the game.
Funny thing, I heard the exact same thing in 2011, then 2012, and 2013, should i keep going...