If I were to do some rough math I'd say it would cost about $300/CAD per month.
My goal is once we are approved to start accepting donations that I can purchase dedicated hardware for this instance. I'd get a used server at about $2300 which would be sufficient a good amount of extra users and through it into its own dedicated shared colo at about $100/month. Factor in about $300-400 a year for drive replacements and we are left with $2300 / 12 month= 191.66 + 100/month for the shared 1u colo + a budget of $400 for drive failures throughout the year $33/month. 191.66 + 100 + 33 = $324.66/month for the first year dropping to about $133 per month after the first 12 months. It's worth noting that this method would give us double the amount of resources and quite a bit of extra storage.
Ideally we don't keep this instance on a single server forever and start to think about spreading it over multiple hosts at or after around 100K users (or less if the number of active users is high).
If someone wanted to host an instance they would not need to allocate as much resources as I have to this instance and depending on how active the instance gets could run off something a lot less powerful.