How is this 3.5 month ROI? Even without paying electricity I get 5 months
What BTC rate are you using. If you calculate the ROI at a BTC rate of $10500 giving you a daily payout for each S9 at ~$23 would give a significantly different comparison at $16500 BTC rate and a daily payout for each S9 at ~$45.
Sorry but he's right. I am not sure where you get the $23 per day figure from, considering we are talking 50TH total, not 5x fully working units.
Even at the higher-end price, 50TH will currently earn at BEST $80 per day before electricity.
So even with $0.00 for power, your ROI is 6months.