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.
Some strange calculations. You should not count from the number of all users, but from the number of users who use the service. I'm sure that a full-scale conversion of all users to a paid service will end in failure for Twitter. Many users, including me, would rather leave the social network than pay $8 a month for a useless tick that an ordinary Twitter user does not need.