When there is money to be gained, there will be abuse. Unfortunately.
Although it often is difficult to determine abuse based on comments only (AI gets better every day), I think you can track down abuse by a 'follow the money' principle.

It's sad that we have to spend time to combat this, but rewards of great initiatives like @commentrewarder should go to the ones that deserve that.

I see @holoz0r has already reacted on this post too. He has dived in comment stats recently. His research may be of some help here too.