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You could query the available add-on products for the current app first. It will return all the products that the user could purchase for the current app. It's a list that contains StoreProduct Objects as items. Then you could find out which add-on product you are looking for and get the price property of that StoreProduct Object.
Here is the document about how to query the available add-on products for the current app: Get info for add-ons that are available for purchase from the current app