I think, the default license for Widgets in the App Store is the MIT license, which allows you to use them in any context and any way, and make modification to them freely, and at your own risk. When you find a widget that you might want to use check if it lists a specific license. Some licenses allow the widget to be used freely in non-commercial projects, but might require a fee for use in commercial projects.
I believe all widgets are free to download. There are modules that you need to pay for.
I don't think you can filter through which are free or paid.
They are not categorized in free or not. You have to look in the descritption if you have to pay for them. But most widgets are for free.