When registering a trademark, additional fees may apply, e.g. if the IP office issues an office action or if an opposition is raised by another already registered trademark. It is not mandatory to proceed with the response to these actions, and the application can also be dropped. However, that means losing all previously paid fees.
If you are applying on your own, the IP office considers it your responsibility to be aware of any and all fees that might be required. If you are filing your application through a trademark attorney or a law firm, you as the client should be notified of all relevant costs that apply to you.
The price for the follow-up legal agenda is usually individual, meaning it varies from case to case. Your legal representative should always inform you about the cost and success estimation before you take any action.