Do you have a low electricity rate?
A petrol car would use around 0.03 gallons per mile (33 mpg). At $ 4 per gallon that's $ 0.12 per mile.
A Tesla model S does around 0.3 kWh per mile. So to get to the same cost per mile your electricity price would have to be below $ 0.40 per kwh.
In a lot of places electricity isn't (much) below $ 0.40 per kwh.