Paid option doesn't really deter this behavior, it encourages it - a botter will see a price tag on a "real" account (see what happened to twitter's blue checkmark sub) and go oh goody, I can pay for people to think I'm real.
If you make the price high enough sure, but I'm unsure you can find the right price to simultaneously 1) deter bot traffic and 2) be appealing to actual users.
If you make the price high enough sure, but I'm unsure you can find the right price to simultaneously 1) deter bot traffic and 2) be appealing to actual users.