Hello,
As explained by @Pascal Slijkerman , a VM that cost 100 $ per month does not inclue the associated costs such as network, public IP, in/out storage and so on.
If you're beginning with Azure and cost management is important to you, keep in mind that once you've created your account, you will be able to configure some Budgets. Once you're logged-in, search for the node "Subscriptions" then "Cost Management", and finally "Budgets". In this part, you will be able to create some specific limitation about your Azure consumption to be certain you will remain in your monthly budget for example - and create some Alerting when you're approaching your Budget.
Kind regards,
Thibault.