1. This would not deter bad actors in any way, spammers already have no issue paying for junk mail. An 0.01 cost means nothing if the action they're taking generates more than 0.01 for them (it generally does). In fact this essentially incentivizes bad actors; you get punished for not profiting off your messages, so people would be more inclined to find ways to monetize their posts.

2. The costs for this would be ridiculous. I have probably sent over a million public messages on Discord in the decade I've been using it. $10,000 is a pretty steep fee to do some chatting.

3. This is essentially a digital ID scheme with extra steps, and requires ceding privacy completely to communicate on the internet.

I understand your comment was probably an off-hand joke and not to be taken seriously but if you think about it for very long it becomes apparent that it would actually make the problem worse.

It costs to mail physical letters, somehow I still get "spam" addressed to homeowner/resident in my physical mailbox.

I think that spammers would happily pay that rate.

I mean...that's how SMS used to work? Or still works?

Once upon a time it was expensive to send messages and now it's cheap.

That would totally amplify the voice of people you want to hear more from, not less /s