Need custimization on Xamarine android application

Hello, i have developed a Xamarine application, i just want to add in app subscription payment system for featured videos in the application so that users can pay and subscribe to access videos that are marked as "featured videos" in the app. I will be using Google In app purchase and PAYSTACK as the two payment gateways. Let me explain a little more: You will not need to build from the scratch. The app has been done and it is fully functional and it is available on playstore. It is a video streaming application. We have normal videos and featured videos in the app. What we can is that, once a video is set as featured video, when a use clicks to watch this video, the video will start playing, but after 5minute the action to pay and subscribe will come up. This will tell the user to "Pay And Subscribe to continue watching, and to access all premium videos". If the user clicks to pay and subscribe for 1month, 6months or 1year package. He will have access to watch for only the time his subscription is valid. After subscription expires he can no longer watch. I need this done ASAP. How much will it cost. Please give me a good pricing.