You're basically asking "What's the net present value of the user?". Let's say that the average user clicks 1 ad per day, and you get 5 cents per click, then e

You're basically asking "What's the net present value of the user?". Let's say that the average user clicks 1 ad per day, and you get 5 cents per click, then each user is worth 5 cents per day. 5 cents * 365 = $18.25 in revenue per customer per year. What's your cost? If you're paying $300k / year in salaries, Red Bull, website hosting and office rent to support a website that can handle 20,000 customers, then the cost per customer per year is $15.00. ...so your profit is $3.25 per customer per year. What's the churn rate? If an average user lasts 2 years, then you're getting $6.50 in total profit per customer. ...except the profit for year two should be "discounted" by some factor (inflation, opportunity cost, risk, etc.). Call the first year $3.25 and the second year $2.50, perhaps. So: a new customer is worth $5.75. You can see all of the variables I used above - feel free to substitute your own numbers in.