@BlackAzizAnansi depends on the number of people actively using it but a large chunk of the resources go into federation so depends heavily on whether by "closed" you mean closed registrations, or closed as in not federating with other servers (this latter is more "discord-ey" and a lot less resources).

I'm running a personal server on my homelab that would probably cost 30-40 euros to run in the cloud (4 cores, 8G RAM, some fast storage for the database, a hosted S3 bucket for storage). I know small servers chugging along in the 1–200 dollars monthly cost range. It does take some of your time to administer, esp. when shit hits the fan like the DDOS attack that hit toot.cafe the other day.

Also +1 for checking out @Bonfire which might fit your intended use case better (I haven't tried it yet but looking forward to it)

@BlackAzizAnansi I have an account at federated.computer that I'm trying to create as a backup ot my slack group.

They're helping me get the slack<->matrix bridge set up, they've been pretty helpful.

Now, federated.computer openly supports some anti-vax chud, and the people who make matrix have a lot of cops for customers, but it's open source and the tech is good.
There's a matrix <-> discord bridge available: https://docs.mau.fi/bridges/go/discord/index.html

@BlackAzizAnansi the masto developers just switched to zulip to replace discord

@BlackAzizAnansi My instance is hosted through https://masto.host/, which has been really great so far. Been on them for almost two years, and the service/support has been solid.

@BlackAzizAnansi If you want hands-off, @fedihost can probably help you with this.

@BlackAzizAnansi Maybe Bonfire would be a good choice?

@BlackAzizAnansi If you're looking to mimic Discord, I think XMPP or Matrix might be better bets? More focused on realtime chatting rather than micro blogging. But other folks would know more!