@Armetron
For many, the increases are not gradual, but rather step function increases in costs, and the amount will vary based on the instance and how they host their infrastructure. For Infosec.exchange, new servers range from $50 to $100 per month each and we have to add another one for about every 2000 active users.

Then there is the CDN. I pay about $300 per month for the service, but it is consumption based.

All told. I would say the costs are about $0.10 per month per active account, but there are many things that can influence it - and also recognize that only about 1/2 of 1% of active accounts make donations.