It can vary depending on the app and what the overhead is, a very small app which doesn't need resource/RAM heavy plugins could run a server for $100 per month, while apps requiring expensive follow and Tag plugins along with full history will need RAM intensive nodes which could start at around $750 pm. This doesn't however cover the cost and complexity to run the nodes from an admin perspective which is often more than the hardware cost itself, also doesn't give redundancy/failover so you have to double that for redundancy, which is important during Hardforks when you will need two servers so you can upgrade and replay one while the other is still running. A managed solution with usage based billing would benefit the community, so you only pay for what you need essentially.