My point was, maybe it is a single number! It's whatever We The People decide, and it doesn't vary: a ton of CO2 released into the air does the same damage no matter what released it. You could spend it saving orphans, and it wouldn't make the CO2 heat the planet any less, and it wouldn't remove the need to find a new way to save orphans that doesn't emit CO2.
Indeed, examining the quote below...
> So you either get more CO2 released than you would with an externality tax or you get less CO2 released than you should given that you can mitigate against that particular CO2 release
...it's easy to miss the point that there's no such thing as "less CO2 released than you should" because the amount we should is negative: not only should we be emitting zero (not net zero, literally zero), and then we must do the mitigations on top of that.
In short, we've tried the existing carbon credit scheme without rationing the available credits, and it hasn't worked to achieve the goal. We need to try something different now.
No CO2 emissions is not the terminal value of the human race. It is a means, not an ends. The damage of that extra ton of CO2 is not infinite.
If releasing a marginal megaton of CO2 caused climate issues that cost a thousand lives over the next 200 years but we want to release that marginal megaton because it let's us save ten thousand orphans than we should release that CO2.
It might be that in the world we live in the correct CO2 per ton tax that takes into account externalities is literally greater than the price of capturing a ton of CO2, in which case all but niche cases (antique car drivers and space launchers probably) will stop producing CO2. But I don't know that and the estimates I've seen have a CO2 tax that pays for the climate externalities at less than a quarter of the price of atmospheric carbon capture (and thats the top end of CO2 tax estimates!).
And my point above was that this is not a single number. It varies depending on what we want to use the CO2 emitting stuff for.