Hello I will buy AntMiner S9 13Th/s

be warned my maths is not an expectation or prediction so do your own calculations and work out how much loss per difficulty jump you expect and form your own numbers

based on lets say antpools 500petahash getting 70 blocks in the last 48 hours(875btc)
which is estimated at 6125btc a fortnight for 500peta

you can then break down that to work out how the maths for a fortnight for just 13thash would result

fortnight income divide 500peta down to 1thash then multiply by 13 thash
(6120/500000) *13 =0.15925

so for the first fortnight, for 13thash would be 0.15925
then expect approximately ~9% LOSS every fortnight due to difficulty jumps and competitor hashpower increases too
the 9% i used is just an example(7%diff chang 2% hashrate change). you should look at the history of difficulty change amounts and then also factor in hashrate increase amounts to get your own fortnightly loss number

0.15925000
0.14491750
0.13187493
0.12000618
0.10920563
0.09937712
0.09043318
0.08229419
0.07488772
0.06814782
0.06201452
0.05643321
0.05135422

this is a estimate of each fortnightly income for 6months which totals: 1.25019621

do not go by some of the crappy online income calculators, which predict value now and simply multiply it by X weeks. ensure you account for difficulty changes and competitor hashpower.

EG if a calculator says you will get say 0.01 a day and then says you will get 3.65 a year. then it is wrong because all it has done was calculate a day then multiplied by 365

be warned my maths is not an expectation or prediction so do your own calculations and work out how much loss per difficulty jump you expect and form your own numbers