This is the wrong way round. Charging people to be popular on twitter would turn it into an advertising platform and drive away the most interesting grassroots users. The benefit flows from the popular 'content creators' to the people who follow but aren't themselves followed.

My own preferred model for charging would be the transferrable 'premium' account: essentially a visible flag on the account that anyone can pay for, like reddit gold. Encourage people to pay for accounts that they enjoy following.