Just Comments is a pay-as-you-go service. You top up your balance with credits which are consumed as soon as you or your readers consume the service resources.
1 million of credits costs $10
Every new account gets 20000 free credits as a trial period.
What consumes credits?
1 comment pageview consumes 1 credit.
What is a comment pageview?
When someone opens a page where the comments widget is included, the widget will request the first 100 comments to be shown to the user. We call this request which fetches up to 100 comments a comment pageview.
You got a blog with 20,000 pageviews per month. On every page you have no more than 100 comments. It would cost 20,000 credits or just $0.2 per month.