I suggest you read FAA EFB Advisory Circular AC 120-76A and EASA Temporary Guidance Leaflet TGL 36.
In addition two observations: at an EASA/FAA level only a class II EFB will allow you to fly completely paperless although a few local authorities have done so on a class I platform as well (LBA). Note that this is being looked at very hard at the moment and might well be reversed in the very near future.
In regards to costs: the business case for any EFB project is driven by e-techlog, journey log, LPC/OPT type applications etc and not by e-charts. Your biggest headache will be your back office solutions.....
