This actually a good argument toward shifting commenting into a paid subscription offering. Give access to the site free of ads and allow you to comment on articles.
It would make more sense to grant commenting privileges to individuals who have engaged with the site previously and are in good standing. Much like the Product Hunt.
Not if getting banned means they have to buy a new account to post again.
I don't know if things are still like this though -- I haven't had a subscription for several years.
On the other hand, trolls and coordinated political campaigns tend to be a lot more willing to spend money to pursue their causes than people that just want to add a relevant anecdote or express their approval, so it might skew the balance of comments more towards trolling and brigading.
You'll almost instantly get downvoted to at least -5, despite the verifiable truth of the statement.
It would make vote brigading really attractive as well. Imagine if /r/TheDonald users could have caused a financial impact to anyone just by downvoting their comments. Reddit would have been a nightmare.
$0.50 is a lot of money in some countries in the world. It'd make trolling vastly more expensive for people in those countries.
If you don't deal with the unpopular/controversial opinion getting downvoted problem it also devalues the unpopular/controversial opinions of those from those countries.
If you're $local_news_paper in some small US town the above is irrelevant, but NYT is read worldwide, as is HN and Reddit. (Tho I suppose Reddit more than HN.)
If you still wanna go ahead with your proposal, I think a good model on downvoting would be to see use it as currency with a relative cost plus an absolute base. E.g. absolute_base + 5%_of_current_karma. The relative cost is to avoid hoarding, the absolute cost is to avoid efficiency of karma depletion.