If you take the deductable tax rate per kilometer, which is 70 Rappen[0], and assume 15000 kilometers a year you get 10'500 Francs per year
If you Google the question you find a variety of answers, but I think looking at it from the tax man's perspective is a good start.
I agree that the cost nay vary, but it can also be significantly higher than 1K per month, depending on the model of car, yearly kilometers and your risk from an insurance perspective.
[0] That's the rate you can deduct from your taxable income per kilometer if you use a private car for work purposes.
