Yoke..
I dont think it is a simple as that. You have the cost of purchasing and maintaining the ipads and the figure i heard quoted for the document subscription was in the order of US$600 per pilot per year. So at a conservative figure of 1000 (600 + 400 per ipad) per pilot, on 2500 pilots US2.5mil to set up and then 1.5mil ongoing costs.
I dont think its as cheap as people make out but, like the newspapers, the electronic subscription rate is dropping over the hardcopy. Unforunately the current document supplier has a bit of a monopoly.