I realize I'm pretty late to the game on commenting here. I don't comment often. I've been on HN for 1558 days and have a karma score of 7. But here are my thoughts.
I use reddit for entertainment purposes (for the most part) and I use HN for educational and inspirational purposes. That was why I started using HN and it is still why I do.
Sure, as the community has grown, there have been more comments that add no value - but they are just more obvious now. It isn't that hard to ignore them and to find the meaningful or useful comments.
On the surface the "cost karma to post" idea sounds great - it certainly wouldn't have much effect on me - but I agree with the various others who think it would discourage unpopular but still very useful and (at times very correct) comments.
If the price to pay to get a well rounded discussion is the presence of vapid commentary - then I'm willing to continue to pay it.