I think payment is a really good idea for deterring bots, and making people behave better too. It does leave a paper trail to your real identity though (unless instances are willing to accept cash or crypto), which many people would rather avoid.

A fixed fee is also relative to who pays it. To a company who only care about jacking up their share price it could be nothing, but for a person working in a very exploitative economy it could be several days wages. I would say that it's the poorest people whose voices are most suppressed, so I wouldn't want to be part of that problem.

Now, having said that, if there was a mechanism for distributing back the surplus from signups after running costs to good users of the instance I would be in favour of it. Per-user instance costs are very low on medium and large instances, so most users would get most of their money back, and perhaps even make money. Making it a cooperative would mean low costs for genuine users and incentivise participation in making it a good instance. If OP wants to do something like that let me know - I'd love to get involved!