In case you already have regular commentator, we don't need to make a contest. Because what we are talking here is the case when we have less comentator, so we make a contest.
In your case, making top comment (best comment) contest is good. And yourregular commentator will be compete in this contest
To be honest, I think most of my regular commentators aren't too fussed about earning $10/month for commentating. They just do it for the love of commenting/connecting and also the free advertising they'll get.
And, I'm always looking for new commentators, as (in my opinion) a blog can never have too many commentators. I just need to think of a way that I can get long term commentators to ADD to my current ones, rather than push them out.