The bots exist for a reason, usually to covertly advertise a product, and by themselves already cost money to run. Someone looking to astroturf their AI B2B SaaS would probably be more willing to pay $10 to post than a random user from a less wealthy country who just wants to leave a comment on an interesting discussion.
History also shows you can take a $10 fee and maintain quality on SomethingAwful for quite some time.
Assuming the money isn't wasted and is actually used to fund moderation 10$ 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.
"my2cents"
0.02 to post or send a message
TLDR: Mail storage is the sender's responsibility. The message isn't copied to the receiver. All the receiver needs is a brief notification that a message is available.
Could we just add complex and varied captcha to the comment & posting forms?
There's just the pesky problem of incentives on the other side of the coin - who gets the $? The spammee? But there would be enshitification issues like:
1. Those who are incentivized to take as big a cut as possible.
2. Those who would put it in their EULA that you must accept their spam and not chargeback or else you lose access to something you value like their services (EULA Ransom... not much different to today "accept our EULA or lose access to what you've already paid for!")
I'm sure there are many other perverse incentives which would creep in..