There is a simple answer to that question. Your app will take (depending on complexity) either three months, six months, or twelve months to build. I've worked on 300 mobile apps over the last seven years. A majority of those projects took four to five months to complete.
Going through the simple time estimate exercise below also allows you to determine the cost of your app relatively quickly.
Let's say your app will take six months to build. You'll need to hire one mobile developer to create the app. The national average salary for a mobile developer is around $100,000 per year plus benefits.
In fact, CNN rated a mobile developer as THE best job to have in America.
Let's do some simple math.
$115,000 (developer's salary + benefits) / 12 months = $9,583 per month
Hiring a developer for your app then equals $9,583 cost to you per month.
$9,583/mo X 6 months = $57,500 total cost
Your app project will cost $57,500 to build.
Key things to remember with any app project:
1.) Only 10% of app projects take less than three months to build.
2.) Developers are very hard to find. Odds are if you find a good one you'll pay more than $100K per year.
3.) The cost of your app continues after launch. (Factor in an ongoing cost of 20% the upfront cost).