got tis formula from a blog... 
guess this is how SE operates...
1.month 10 person invest 10k. he got 100k first month.
2 month- 20 person con and get 200k, he pay out 1k to the first 10
person
3 month- 30 person con and invest 300k, pay out 30k to 30
person
4 month- 40 person con and invest 400k, pay out 60k.
Now, he got 1000k and he pay out 100k. he still have 900k.
5 month- 50 person con and he get 500k, pay out 100k
now he got 1300k
6 month- 60 person, get 600k and pay 150k
now he got 1750k
The more people coming in, paying u 10% per month is nothing to JP.
what he gain is even more.