Need Help To Create An Extension

Hey readers how are you doing ?

I need help from you guys. I want to create a extension for Mobile Phone Prices. I mean the extension that can tell people easily to know; what is the worth and current market price of that phone. But actually i am confused; how to start and which country to follow how much it would cost. And what i am worried about is how much shall i spend and what would be the return ?

what you guys would suggest to shall i invest in this project or not ?