If CO2 reduction was the only goal, then yes, that would be a very steep number. That's obviously not the case though.

The cost to tax payers isn't a hit to the overall economy though. The government spending replaced fees that would ordinarily be payed by taxpayers and tourists.

If saving CO2 were the only goal, sure.

Right. I wrote that in the context of the linked article.

So don't think of this scheme as a reasonably efficient way of saving CO2 emissions (there are other much more economically efficient ways); at most it's a bonus.

It follows that it doesn't make sense to justify the scheme on the CO2 emissions benefit. I've seen a lot of that on European subreddits.

But with global warming, that should be almost the only goal for an environmental policy.

The nine euro tickets weren't an environmental policy. The environmental effect is a bonus.