Software-Engineer & StartUp fanboy licking the shiny little boot of IT. Holding a German patent for mobile location-based tech. Speak German, English, French & Polish
While you surely have made sure, that you are solving their #1 problem instead of #24 I'd suggest you check out Rob Fitzpatrick's book "The Mom Test" which will help you easily review your questions and strategy: http://www.amazon.com/The-Mom-Test-customers-business/dp/1492180742
This should neither use a large amount of mobile data nor use as much battery as Gmaps. Can't speak for iOS but this is what the Android documentation below has to say on location strategies and what it boils down to is that you need to determine how much accuracy your app needs, pick your strategy and maybe implement it as a background service that talks to your app. Someone who coded for Android before should have a prototype up and running within a few days. I assume however that this involves extensive testing on various devices and OS versions (the tricky part of mobile development).