That's a pretty cheap price for Antminer s9 pro if you can get it for $6,350 each. Just make sure that you are dealing with a trusted distributor or seller.

About the ROI it will still depend on the Bitcoin price and difficulty since the difficulty drops well it still worth having those gear and hold your mine coins for the next block halving.

According to some tools, the ROI is 2 months and 15 days to reach the ROI you can check for yourself and use this tool below

- https://www.mycryptobuddy.com/BitcoinMiningCalculator

Just include the hardware cost in total and the power consumption including the total hash rate.