Hi Andrei,
When you choose to use your own virtual network several managed resources will be created automatically, and you are charged for those (as you noticed). One option is to use automatically generated virtual network instead of your own, but you will not have as much control.
If you want to switch to different VNet configuration you will need to create from scratch since you can't change it after creation.
Please see article below for full details:
Networking in Azure Container Apps environment
https://learn.microsoft.com/en-us/azure/container-apps/networking
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Thanks.
-TP