The cost of a commenting system is not in hosting, it’s in moderation.

And that cost falls squarely on the site owner not the comment service. Moderation may be as expensive or as cheap as you make it.

Big Tech is spending a lot of money on it, thousands of people and jobs and algorithms.

If you ask me, it's a fools errand and I hope that they waste their money trying to build a clean Internet.

Reference: https://www.remarkbox.com/remarkbox-is-now-pay-what-you-can....

I don’t know the rules for Disqus but I expect that they impose at least some limits to block bots and spam, which is almost impossible to achieve for a small site.

Check out akismet, it works for wordpress and is likely all you need to divide spam from ham during moderation.