Ben's numbers agree with what I've seen. To calculate your cost, multiply your miles driven by 0.336 kWh per mile (the actual number we're seeing from wall-to-wheel) then by your cost per kWh of electricity. We get a little better efficiency for our 2002 Toyota RAV4-EV, so probably other EVs will be in the same neighborhood.
Cathy and I pay the $0.0125/kWh premium for "green energy," bringing our total cost up to $0.115 per kWh hour. So our cost for 12,000 miles per year would work out to $464 for the Roadster, $442 for the RAV4-EV.
Before we got the RAV4-EV and then the Roadster about a year later, we drove most of our miles in a Honda Insight (52 mpg, $692 per 12,000 miles), most of the rest in a 1996 Nissan Pathfinder (17 mpg, $2,118 per 12,000 miles), and occasional nice-weather driving in a 1995 Acura NSX-T (21 mpg, $1,714 per 12,000 miles). The NSX and Pathfinder have moved on to new owners. We'll probably sell the Insight soon, then we'll be pure EV.
So it's not really how much does your electrical bill go up, it's how much does your total transportation energy bill go down.