Hey @flarum-türkiye are planning to use Cloud-Infrastructure?
This really depends on the time each user spends on your site.
An EC2 m4-medium in AWS with about 18K Users spending 15min each day is about 100$ per month.
The MongoDB cloud with 25GB is 25$ per Month.
Soo, there appeared to be two errors. One with the nginx config which I solved in the past ( a / typo behind the proxy_pass ) for anyone having this same issue make sure you double check the nginx rules for a typo.