How much an Android Store app cost me?

Hello, I am running a Shopify store, I want to have an android application for my store. The app should have all the features of adding, removing and updating the cart and a payment accepting method. What will the app cost me? and usually how much time a developer will take to complete the project. I know it is not right platform but I am seeking information so I can communicate well with my developer. Last thing Unity will be good platform for store type apps.....??

This should have been your first thing because the answer is "absolutely not."

Would you suggest me a cross-platform compatible for IOS and Android.

Shopify or accessing Magento's REST API.