Have a listen to the podcast "Transport Evolved" - the host and guests often speak of the experiences they have with the new Leaf, and other alternate transportation. I suspect that you can do the math yourself using the appropriate formulae, given the somewhat known cost of electricity.
Do the math yourself, but I recall it came out to somewhere in the neighborhood of 30kwh for a full charge, so for the typical 100 miles - it costs in pure electricity: 3.33 miles per kwh - = 0.15 per kwh = $0.033 per mile. So if you charge empty to full every day - it would be somewhere around 900kwh extra per month - or $135 to go 3000 miles.
Thats not the total cost though since you need to add in the cost of batt maintenance and the charger itself and any away from home charging too.