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.
@julian the issue was different mongodb versions on different servers, when the database migrated on the different version of mongodb it failed with that error periodically. The odd thing is it did not crash, just hung threre.