Do you know of any research or calculations of what that number ought to be?

If you wanted to pay for direct air capture of CO2 to directly "undo" your climate effect of driving, the cost would currently be about $6 per gallon. Price comes from [1], found [2] looking for a second opinion on current direct air capture cost.

[1] https://theclimatecapitalist.com/articles/gas-should-cost-13... [2] https://www.forbes.com/sites/phildeluna/2024/11/29/will-dire...