I think you underestimate the probability that the world does not divide neatly into trolls and non-trolls. Rather, the troll lurks within us all, and without constant vigilance, under the right conditions the troll will emerge!

In other words, users will not pay to troll. They'll pay to make a comment, which may or may not be a trollish comment. (A lot of trolls can't, in the heat of the moment, tell the difference between a troll and a comment -- that's what makes them prone to being trolls.) And then they'll have paid, which means there's no barrier at all to future trollish behavior from the same person.