Carbon offsets currently cost under $20 per tonne. A litre of gasoline probably results in around a kilogram of CO2, so the cost to offset the CO2 in a litre is around $0.02, i.e. around $0.07 per gallon. I don't know if it's so easy to quantify the other consequences, but we're likely talking about amounts that are measured in dimes, not dollars.

So why increase the price all out of proportion with the consequences? I'm all for cleaning up my own mess, but if my mess costs dimes to clean up, why should I pay dollars to clean it up? Gasoline isn't inherently evil, it just has some bad consequences.

Why not make sure that carbon offsets have already been purchased before any gasoline gets to the consumer? Then you can use as much as you like, and it's all carbon neutral. Work out similar offsets for other pollutants and make gasoline completely green.