Check in to locations App

I want to create an app that utilizes maps and GPS so that people can pinpoint their location and share this with their friends (signup through an account or facebook). Choose when you're at the location (either NOW or from a time). This location is then visible to all other friends with the App (and on Facebook) If someone is in proximity (within 10km) they are notified of this person checking in. They can then reply quickly with "coming soon" or "come to me I'm here!" When you check in to a place you can either add it or review and take photos of it. You can see on the map where your friends have checked in to (which have an auto expiry) Simple interface and design. Just want to understand costs for development (design to be provided). Android, iPhone, Windows would be awesome - but starting with iPhone would be good. I will also work on developing a website to couple with this. Thanks, Ben