Twitter's mMAUs (monthly Monetizable Active Users) stands at 450 million. If they pay $8 per month, that gives Elon $3.6B per month. It will take 13 months to make $44B - the purchase price QED. Elon Musk is a genius.
It would be interesting to know how many of those verified users are staying with twitter and how many are leaving. I would guess though that the overwhelming majority will stay and then you are absolutely right with your statement that this was a great deal by Elon Musk if everything works out as planned. I think though that those verified users don't really have a choice here. I would guess that they make their money with twitter and therefore they need to be verified and there is no other platform that has the same reach as twitter. I know that mastodon is becoming a little bit more popular at the moment but it is still nowhere close to twitter.