I would probably not pay $10 to post on HN, but many spammers who expect some kind of tangible return would pay that, so the fee just makes the problem worse.
Assuming the money isn't wasted and is actually used to fund moderation 10$ is probably comfortably above the cost to detect and ban most malicious users.
There are large swaths of spammers that indeed would not pay it. There are on the other hand plenty of NGO's that would pay it without a second thought to promote specific topics and dogpile on others. Those are the movements I would expect AI to take over if not already. AI does not sleep, humans do. AI won't miss the comments that groups believe need to be amplified or squelched.