The money is never really on the front-end, it's on the backend. Hence the ability to pay out huge % commissions on front-end product sales to affiliates, JV partners etc. For most of the big products on clickbank, the product owner is going to pretty much look to break even on the front (anything better is a bonus), or maybe even take a little loss - and then up or cross sell a bunch of stuff on the back end - other products and courses they own, JV partnerships etc etc.
You start out, get data, optimise and tune your funnel and get to a stage where you break-even (or can afford to lose a bit) on your $40 front-end product, but you know your average customer value is say, $150 on backend funnel promotions over say 4 weeks of carefully pre-selected offers and addons after their initial front-end product purchase.
Here's a good interview Tim Ferriss did with the truth about abs guy:
http://fourhourworkweek.com/2011/11/02/the-truth-about-abs-mike-geary/
Although not specific to ebooks (they have a lot of info products in their funnels though), these are some very good reads that are relevant to your question:
http://www.digitalmarketer.com/how-to-calculate-average-customer-value-funnel/
http://www.digitalmarketer.com/customer-value-optimization/
You start out, get data, optimise and tune your funnel and get to a stage where you break-even (or can afford to lose a bit) on your $40 front-end product, but you know your average customer value is say, $150 on backend funnel promotions over say 4 weeks of carefully pre-selected offers and addons after their initial front-end product purchase.
Here's a good interview Tim Ferriss did with the truth about abs guy:
http://fourhourworkweek.com/2011/11/02/the-truth-about-abs-mike-geary/
Although not specific to ebooks (they have a lot of info products in their funnels though), these are some very good reads that are relevant to your question:
http://www.digitalmarketer.com/how-to-calculate-average-customer-value-funnel/
http://www.digitalmarketer.com/customer-value-optimization/