My first thought was absolutely yes but then I began to wonder whether that would do anything for the quality of submissions. $5 isn't very much; why would that discourage low-value comments any more than high-value ones? I assume you mean that reading would still be free but you would have to have a paid account to post?

Counter-proposal: make it easier to tie someone's HN name to a real identity. Perhaps require email address confirmation and a waiting period for new accounts before allowing a first post?

On the high-ratio orange-username thing: neat idea but comment karma depends so much on how early in the life of the thread a comment is posted. Readers who check in less frequently and respond to longer threads are less likely to score high regardless of comment quality. (Also, I'm sad to not be orange.)