Just multiply 85kWh * your electric rate. If you get the R-OTOD plan, it looks like your off-peak rate would be about 11.3 cents/kWh, so you'd be about $9.60. Plus maybe 10% as Todd points out below for charging losses.
I think a more useful comparison is to calculate monthly fuel savings based on how far you drive. Here's an example from Plug In America's "Electric Driveway" party kit:
Tips on Calculating Fuel Savings
Most electric passenger vehicles will travel 3-4 miles per kWh, depending on efficiency and driving style. The US average kWh costs 11 cents. So a plug-in vehicle can travel a mile on 2.75 to 3.7 cents. The average driver drives 1,000 miles per month, and so will spend between $27.50 and $37 on electricity each month.
The average gasoline passenger vehicle gets 22mpg. The US average cost of fuel at the moment is $3.50. So gas costs 15.9 cents per mile, which would be $159/month.
The average driver would save over $140/month in fuel costs if they switched to electric drive. That’s in addition to savings on maintenance, oil changes and emissions inspections. And based on past pricing trends, gas prices are likely to rise significantly more than electricity prices, so the savings are likely to grow.
To calculate this for a specific instance, you need to know
• The price of a kWh of electricity from your utility
• The price of a gallon of gas
• The MPG of the old car (see fueleconomy.gov)
• The kWh/100 mile rating of the plug-in vehicle (fueleconomy.gov just started listing these too)
• How far you typically drive in a month
Plug these numbers in to the following equations (with average samples given):
Gas price per gallon / Miles per gallon = Gas price per mile
$4 / 22mpg = $0.182
Electricity price per kWh * kWh per 100 miles / 100 = Electricity price per mile
$0.11 * 34 / 100 = $0.0374
Gas price per mile – Electricity price per mile = Savings per mile
$0.182 - $0.0374 = $0.1444
Savings per mile * Miles driven per month = Dollars saved per month
$0.1444 * 1,000 = $144.40