Pay per view for questions or answers? To whom you are going to pay? If you have a question with 5 answers with each answer on different months, how do you calculate views for each answer?
If there is such a plugin then you can use it as a multi-author blog in q2a. Like adding a title on the question and question body as an article body. Then you may disable answers. In such a case you can make use of such a plugin. I am also interested in having a plugin of the kind.