Scam bot operators will just use stolen credit cards - or even easier, iTunes gift cards that they get from the victims of their scams - to pay to "prove" that they aren't bot accounts. For the fake followers/interaction bot "services" it increases the cost of operating, but I doubt they spin up a bunch of new accounts for every client - that $1 per account can probably be spread out pretty thin. I don't see this solving the bot problem any more than prioritizing paid account replies did (it didn't work at all for that).