ROI 
? newer 
AntMiner S5+ Batch 1 7.722TH/S 2375 USD 11 kg x 1 8.905 BTC
( 2375 USD )
0 BTC
(0 USD) No Coupon 8.905 BTC
( 2375 USD )
Shipping Cost 0.787 BTC
( 209.872 USD )
Total 9.692 BTC
( 2584.872 USD )
? newer AntMiner S5+ Batch 1 7.722TH/S 2375 USD 11 kg x 1 8.905 BTC
( 2375 USD )
0 BTC
(0 USD) No Coupon 8.905 BTC
( 2375 USD )
Shipping Cost 0.787 BTC
( 209.872 USD )
Total 9.692 BTC
( 2584.872 USD )
here's your RIO Answer.
What is the monthly growth of difficulty? Zero? If yes then calculations are not useful.