You can take this further by providing different prices for various initial point scores of a submission say from one to five with five being five times more expensive than one.
Subtract 1 karma from yourself each time up-vote or down-vote is used; this effectively turns karma into "money" that can be spent on maintaining good discussion and high quality submissions.
This would make people think twice before down-voting. It would also reduce "knee-jerk" up-votes, as people would be more likely to reserve up-votes for things that truly deserve it.
It would also mean that long-time users will naturally do more of the moderation, as they have lots of points and may not be as afraid to use them.
(And a little inflation should not hurt too much. We have a lot of inflation on HN right now.)
A bad side effect could be that HN users might be reluctant to post interesting items that were only likely to appeal to a minority of their fellows. Technical items on web development rarely get more than three or four up-votes and yet many are likely to be of interest to web based start-ups - part of the core HN target group.
Does not feel like a net win to me.
You would think someone would have figured out an effective method of user-generated content quality control.