What type of app are you building?
Apple iOS is a better choice to reach a more engaged user base. Android has a broader reach, however, particularly in emerging markets, like Asia and Africa.
Do people have to login?
An email login is generally best to start with unless your app will have tight integration with services like Facebook or Twitter, in which case social login is better.
Do people create personal profiles?
A profile means that some of the people that use your app will have to enter information about themselves that might be viewed publicly.
How will you make money from your app?
Upfront cost is cheaper to build than in-app purchases but in-app purchase can produce higher returns if/when you have an engaged user base.
Do people rate or review things?
Keep in mind, ranking systems can vary in complexity and thus impact your budget.
Does your app need to connect with your website?
This means you'll need to make an API (or Application Programming Interface.) It's how all your friendly apps talk to each other.
How nice should your app look?
Custom gestures, custom buttons, and custom transitions will cost you — it's not cheap being pretty.
Do you need an app icon?
Designing a quality app icon typically ranges from $500 - $2,000 but will help you stand out in the app store and on the device.
