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.
I'm using whattomine. I believe the price was $10,500 on whattomine when I posted