Do you happen to know what are accurate numbers? I'd be pretty interested in knowing what are the actual numbers for reddit's hosting costs / active users / cost per user...

I really don't know, but I share your interest. I'm not sure if it's out there at all, since they're a private company. I think the guesses that we have mention that they're based on extrapolating data that was somewhat questionable to begin with, as it came from someone with a clear interest in showing high numbers of users and income, but with no duty to report either of those truthfully. If you read Christian's post closely, he inflates both users and income to favor /r/ and give them the benefit of the doubt.

My other mention of one penny per user came from another thread where an admin of a smaller instance was saying they received a surplus of donations, and that they told their user base that a single penny each month would suffice. I suspect lemmy.world has a higher base cost to run, as he has mentioned he has proximity to this infrastructure and the means to run there at small scale with no financial support. All that is to say that I think some instances with <10k users can run on cheaper hardware, but @ruud was able to start over provisioned and expected to see >10k users. He also shared a cpu utilization chart showing his instance never approaching even 20% load.

I'd guess that at present active users, depending on instance plans and needs, server costs can run from between 1 and 15 cents per user.