Back of a fag packet calculation:
We have a big nav bag with Jepps, two SEP manuals, and a few other docs. Elsewhere in the flight deck you have the MEL, 2 FCOMs, QRH, Ops Manual. That lot has to be at least 10 kgs.
At 2.5% per hour that's 0.25kg of fuel per hour just to carry it around. If the aircraft is flying 15 hours per day, thats roughly 5500 hours per year and 1.365 tons of fuel per year.
Latest IATA fuel price is US$1092/metric ton. 1092*1.365= US$1490.58.
So by not carrying around our paper docs we would save about $1500 annually, that's over $300,000 for the whole fleet.
Obviously you have the cost of purchasing, maintaining and carrying the tablets to take off those savings.