It depends, there are phases and steps you should be do if you are taking your app idea to any investors.
The first thing is how much money are you willing to lose? Investors hear and see pitches all day long so is your going to wow them? You need to remember this you are not just building a mobile app, you are starting the structure of a new company. If no one invests in the app can you afford to bootstrap it?
If you are wanting to pitch the idea of the app and have a working prototype, you should have it built completely functional, the app would be so close to being done.
Without knowing what your app idea is or what operating system you want to start with I can say it may cost you anywhere from $500.00 and UP… Always get a NDA signed before sharing your idea..
This will save you alot of money and time. Go to google documents or google presentations build yourself a presentation, step by step, PowerPoint formatted, include page by page function and graphics?.
You will want to have it done to make the app built anyways, if you want to save money.
Let me know if you need something else or want to talk.