What's a scenario in which Musk will at least recoup his acquisition cost? Even if 10% of Twitter users (and that's a lot) will get on the 8$ plan, revenue is only roughly (100*20M?) 2B$ per year.

As I make that napkin calculation I cannot imagine actually 20M people being active enough on Twitter to justify paying 100$ per year for it. Especially if the price won't be catered to regional purchasing power.