Wel... when you advertise you need to take into account the lifetime value of a customer. How long will (on average) a customer stay with you? 1 year? 2 years? 3 years? More?

Say 1 year. If your average profit per customer is (lets say) $10/month then a customer is worth $120/year. You could spend $60 to acquire a customer and still make $60 profit per year. This is why some affliate programs pay $80 per sale. because in the end they will make a profit. (Sure, some of them corners too. )

Anyway, that kind of estimation is easier to make if you've been in business a while and you have a sufficient number of customers to draw some statistical conclusions. Otherwise it is a bit too risky.