I was curious what it would actually be. Assuming a full offset with expensive direct air capture of CO2 ($238 / tonne [0]), the carbon tax would only be $2.12 per gallon for 8,887 grams CO2 [1].
Capturing the carbon is only part of the cost... The other part is sequestering it for millions of years. Concrete is a great place to do that, where many other 'sequestering' techniques might only last tens or hundreds of years, requiring the capture fee be paid again and again and again.
[0]: https://www.iea.org/commentaries/is-carbon-capture-too-expen... [1]: https://www.epa.gov/greenvehicles/greenhouse-gas-emissions-t...