$8 should be plenty sufficient to cover some sort of identity verification service -- assuming possession of a distinct US credit card number isn't sufficient on its own. (Could make it so if you mouse over a checkmark, it displays a flag based on what country the payment for the account originates from.)

Additionally, I don't believe bot detection is an inherently hard problem. And if every bot detected means the bot creator has to pay another $8 for their next bot, then bot detection becomes profitable for Twitter.

I doubt this ends up being the cheapest way for Russia to influence US elections.