Agree that cashflow is a different kind of resource than time and energy. But for the record, here are my 12 month (cash) costs for the larger of my two small forums:
- domain (namecheap) $14.16
- hosting (digital ocean) $72
- mail service (mailgun) $15.74
which comes to just over $100 (although I paid in pounds sterling).
Another thought about cost: if you might in future need to shut down your forum because you can’t afford it, that’s a strong negative for your community even from the beginning. You need to have commitment to keep the thing running. So be prepared to bear the cost, or be explicit about the need for an annual funding drive, if that’s what will be needed.