Airtable is rolling out a new free plan where you can use one app per base. You can see more info here. If you choose an app that was created by Airtable, there is no additional charge. If you want a 3rd party app, some of them have additional costs, which varies based on the app.
If you want to use Scripting App, it is included at no additional charge in all plans.
Is there a particular app that you are interested in?