The costs will vary wildly depending on how the instance has been set up. If you set up all necessary services on a single VPS (as is the most common approach for smaller instances), then you can probably get by on $10-$20 a month. Splitting different services onto different servers, adding backups, load balancing, CDNs, redundancy, caches, etc will quickly increase the cost. Bigger instances need more powerful servers, that will increase the cost further.

On lemm.ee, we are currently not using very high-end servers, but we ARE using all the other things I mentioned above, and the monthly cost is currently hovering around $200 (that's for 3 servers, a managed database, object storage, load balancing, a global CDN, and an e-mail provider). This is still on the very cheap side in the grand scheme of running online platforms, but definitely much more than I would want to pay for a single-user instance for example.