999 karma · joined June 18, 2008
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Find out the average number of purchases they make.
Multiply to get average lifetime value.
Spend less than this number on your customer acquisition, ie Adsense.
Example: The average subscriber stays with you 1 year. It's a monthly service, so the average customer makes 12 purchases. You have a low/no cost service with a gross profit of $2/mo.
12 x $2 Profit per Unit = $24 LTV of the average customer
If you have some sort of viral component where users are getting other users, you'll want to account for that as well.