There is a fundamental difference between Azure Function Plans and App Service Plans.
To explain this, we currently have multiple options to host Azure Functions:
Every of those options comes with different features and costs. But we can break them down in those 3 categories above. When you want the most Function and Serverless like features, you should go with the "Serverless" group. There is a good comparison in the docs when to use what.
For the pricing, please see the pricing pages for functions, app service, container apps
There is some documentation regarding deploying on App Service Plans. However, I'd recommend you to use the "Serverless" options, which provide more features and you do not need to setup scaling rules by yourself. Maybe even consumption is an option for you, which brings also pricing benefits for infrequent called functions.
If you want to shed more light on your requirements, we can talk what plan is the best option for you.