It's a little hard to nail these things down exactly. Usage varies from month to month. One complicating factor is from time to time you get free charging for the electric car. To date no one has ever handed me free gasoline.
I charge at night. Our lowest TOU tier is $0.12 per kWh, including delivery, taxes, etc. Gas prices here currently float around $1.20 a litre.
My former Infiniti G37 averaged around 8 litres / 100 km, or about $0.10 a km (with my particular mix of city and highway driving).
I estimate my Roadster cost me $0.03 a kilometer (again, with my driving profile).
The Model S is larger and consumes roughly 30% more power, so that's $0.04 a kilometer.
Based on that and my typical mileage per year, here are some estimates:
Infiniti G37 $160/month
Roadster $50/month
Model S $65/month
Estimating this a different way (average power bill before and after getting the Roadster), I came up with $30 to $40 a month for my Roadster. At first I was surprised that this method came in so much lower, but then I realized this is probably due to road trips. My estimate above is based on city driving. My road trip consumption is generally a lot closer to "Ideal", as I'm trying to maximize range. On top of that I've yet to pay for charging while on a road trip, so it only costs me whatever it takes to top back up when I get home! A typical 1,000 kilometer trip costs me $5.