Yes, to create custom TLS/SSL bindings or enable client certificates for your App Service app, your App Service plan must be in the Basic and above tier ( Standard, Premium, or Isolated tier). It is the ASP (App Service Plan) tier.
Within an ASP, you can provision unlimited* WebApps.
-Since you pay for the computing resources your App Service plan allocates, you can potentially save money by putting multiple apps into one App Service plan. You can continue to add apps to an existing plan as long as the plan has enough resources to handle the load.
You don't get charged for using the App Service features that are available to you (configuring custom domains, TLS/SSL certificates, deployment slots, backups, etc.). The exceptions are: App Service Domains, App Service Certificate, IP-based TLS connections.
Here - B1 is the ASP and you're correct it is 46.17 EUR/month, for the plan- You can host unlimited* apps under this - see [pricing details] for more details.
When the resources are not utilized, you can scale up and down at any time. It is as simple as changing the pricing tier of the plan. You can choose a lower pricing tier at first and scale up later when you need more App Service features. You can even pause/stop the App Service plan to reduce the cost based on your business requirement.
Just sharing additional info:
You could also set URL rewrite rules to redirect traffic away from the internal domain name and directly to your custom domain name. I believe you have already done this, kindly review the URL rewrite rules, kindly check this sample.
