A good head for math?

[raymor]

(Avg number of months retained)*(%payout)*(Monthly member fee)

What common variable you forgot is processor fee.
Many, if not most programs advertise a % payout
of the net sale _after_ CCBill takes their 12%-15%
or whatever, so (%payout)*(Monthly member fee) does
not equal your payout. Instead it's:
(%payout) X (Monthly member fee) X (100 - %processing fee)

Then of course you're multiplying by % twice in the above
formula,e so you have to divide by 100 twice to get back
real numbers, so it's:
(%payout) X (Monthly member fee) X (100 - %processing fee) / 10000