The figure below indicates where savings are coming from. Though the study comes out of Canada, I believe the currency quoted is US$ , though I am not 100% certain. There are other studies that came up with similar order of magnitude values.
In the article the authors explain that the grid benefit wouldn't only be attainable with bi-directional charging ability.
Interestingly, in the context of heightened national security concerns over recent times, the "National Security" benefit exceeds the "Health Impacts" in this study... by a factor of 2[!].
For every EV not bought because of the EV only road tax, all of the above benefits are forgone.
