Web app is not billed directly but over App service plan.
So, you pay for App service plan and web app is placed under it. Also, you can have multiple websites (depending on tier of your App service plan) under same App service plan and you'll pay only for one service plan, not for each website separately.
For instance, if you have 10 websites and all of them are under one App service plan (let’s say it’s S1), you’ll pay only pay once for your service plan (around $75/per month for S1). When you create service plan, you will pay its full cost per month, no
matter if one web app is running there or 50 of them.
Mustafa Toroman, Azure MVP