Paying for a million fake users would be peanuts to a state actor. Russia, China, Saudi Arabia, etc... These users post on average 2 tweets per hour (my personal observation, not a reliable data point). A million users tweeting twice every hour that's 1.5B tweets. Per month.

I agree that introducing paid accounts would make it harder for content farms to exist - primarily due to the fact that they would need a very large number of bank accounts and/or payment cards. However I don't think it's a real solution to this problem. Maybe as a part of some holistic anti-bot system? Yes, that could work.