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's not that easy though, not sure if everyone is willing to pay that much.
But obviously, this is the business side of why he wanted to acquire Twitter itself, to make more money isn't it. Maybe it will not be just a year that he will get his money back, maybe it will at least take a couple of years. And not sure if mMAU = blue badge, because that is what the initial news is. They are going to charge for those who are verified and have that blue badge under their names.