The cost of producing the object (Fixed, or marginal) being sold isn't particularly relevant to what you should price it at.
For example, check out this dandy PDF Report: "The Networked EV: The Convergence of Smart Grids and Electric Vehicles" [1]
Single License: $2995
Price to maximize long term profit. In some cases, you may want to build your readership by reducing the profit on the current sale - but, for a solid agile development book, presuming it is solid, $99 is a "friction free" price for most agile managers - they won't hesitate to buy it if it's a best of breed. Those that would hesitate at $99 would probably think twice at $50 as well, so there isn't a heck of a lot of incentive to drop your price down that far. If anything, would have probably suggested something like $129, but $99 probably gets the job done.
Anything below $50 is silly, because at a certain point, your audience starts to think you're offering a lower quality product, and you start to _lose_ readership.
[1] http://www.gtmresearch.com/report/the-networked-ev-smart-gri...