Would you pay $12 per year to have Slashdot? Browsing would be free, but how about $12 per year for moderation and commenting permissions?
That's less than most any magazine. I subscribe to Nat Geo and Smithsonian and $12 per year is less than either subscription. And I spend more time on Slashdot than I do reading both combined.
Just throwing it out there, $1 per month may not be the right number, or even needed.
I bet this would help with trolls and very offensive comments as well. I've only seen the bottom of the bucket a couple of times, it is nasty down there.