One small matter though, $100m will not get you much public transport.
I don't expect it to. It would have been clearer for me to say, "for
each $1 a tonne of emissions, you get $100 million of revenue." It was also to illustrate that each 1/3 cent extra per litre on petrol/diesel could raise $100 million, so that a relatively small tax could raise a significant revenue. Here petrol prices vary quite a lot. Just in the last 45 days the average price around Melbourne has been as low as $1.47/lt and as high as $1.67/lt. So a carbon tax equivalent to even 5c/lt (~$17/t CO2e) would be lost in the noise of day-to-day variation of prices. And that's $1.5 billion.
Really capturing the economic value of the emissions, honestly I think a fair rate would be $1,000/t CO2e. Which would make petrol $3/lt more expensive, about $4.50/lt. That'd be $160 for a tank even for a small car like ours. Since it enables you to earn much more money than that it's actually reasonable. I mean, just for the petrol and diesel burned that'd be $100 billion, compared to our economy of $1,330 billion - is all our driving to and from work and trucking things around worth at 7.5% of the economy? Well, stop all cars and trucks overnight and see what happens to the economy, it'll decline by a lot more than 7.5%. So, $1,000 a tonne is actually reasonable. And $100 billion would build a lot of trains and solar panels and all that.
However, imposing this level of tax overnight would of course absolutely devastate the economy, like tripling minimum wage overnight.
A more sensible approach is just to start with a token tax, and have it enshrined in law how it'll rise each year. Just as we did for tobacco tax, we keep raising the rate until we get the outcome we want - a low-carbon economy. If you keep the tax the same then behaviour doesn't change much, there's a small change and then people and businesses adapt. You want to keep forcing the change, which means a rising tax.
However, currently governments of either of the main sides of politics are forever promising to cut taxes, so imposing a new tax and committing to raising it every year would go completely against that.
Likewise, simply imposing a carbon tax would just mean we import more carbon-intensive stuff. Let's say you tax coal when it comes out of the ground, guess what, we'll be buying foreign coal instead of exporting it. Which means even more money flowing overseas. Thus, in the absence of a world carbon price, we'd have to put a carbon tax on imported stuff. But that's tariffs, and that's prohibited by the WTO. And governments of either of the main sides of politics are pro-free trade these days, and carbon tariffs would go completely against that.
So I don't think it'll happen. Central governments are failing to take necessary action, they're too busy with trivialities. It has to be state/regional, local and individual. Like in Australia, while the federal government was arguing about whether to keep the renewables target for 2030, the state governments went ahead and will exceed it by 2020.
Forget about Canberra, Washington, London, Paris, Tokyo and Berlin. They're busy rearranging deck chairs on the
Titanic. Get in the lifeboat and start rowing.