One of the benefits of using Realm Auth Providers within your application is the ability to enforce user access to your services/data access by defining simple and declarative roles/rules which can be automatically map to your user context.
This ability will not be available if you use Realm Application just for authentication and then a regular driver to connect to Atlas.
Do you mean you would use Realm Authentication just to get an OK for the provided credentials? If this is the case this is possible but I would recommend considering using Realm scalable and secure backend as a full solution. Moreover , that we have offline sync capabilities for mobile reactive application developments.
Realm has a pretty large free tier range and the cost can be extrapulated by using the following formulas:
Free tier metrics are zeroed every for each month:
Can you elaborate your concern on getting a large bill? What is the method you used to calculate that?