To Hubskify this idea, the longer and more pinwheeled a comment is, the less it should cost.
That's far too simple. I want to see grammatical analysis applied server-side so long, complicated, information dense posts are forced into existence by economic pressure and low effort one-liners like this cost me a dollar!
An arbitrary value. If I were going to do it for real, I'd set a range of costs between $0 and $Something where $0 is the Gaussian analysis of the length*grammar score of highest voted, densest comments in the entire Hubski database and everything else falls away to $Something that would be enough to make one think twice about being glib and lazy. A Thoughtful Web Is Free; For Everything Else, There's Mastercard. This would still be costing me money.