Mostly it was just the cost of the domain since there was free hosting all over the place. You could also just run it in an old PC in your bedroom, not sure how much electricity that used as I was not paying electricity at the time :) but mostly the problem is simultaneous users, 100 users is ok as long as they are not all making requests at the same time. Anyways, with modern frameworks it would be practically free.

But the SEO and marketing costs to get any kind of traction these days are in the hundreds of thousands of dollars. That’s the real reason we can’t have decentralized communities anymore, the big guys have formed oligopoly. They control the market and prevent any competition by owning user attention.

I don't know what the lower bound is, but a specific answer is "$97.67 per month" which is what I pay for a server in a rack in a data center that has been chugging along, with an occasional drive failure, for the past 15 years that hosts a forum, a bunch of low traffic websites, and an email server.

I also run a BBS on a Raspberry Pi that plays host to a few hundred DAU, that engage in chat, forums, a MUCK, and the occasional file transfer which runs on my home internet connection.

I am sure someone will jump right in and tell me that I am doing it wrong and I can host everything on an AWS micro VPS for $4/month, but the amount that I pay is for my server. I also suspect someone will jump right in and tell me I am not allowed to host a server on my home internet connection.