It depends, what is the app actually supposed to do?
I don't want to explain everything right here.
I'm not expecting an exact figure. I can tell you it's simple to make. It doesn't even need to be pretty or polished. It's for my personal use and not to break download records.
If you're a developer can I see any of your previous jobs?
It would be at least 1 BTC, even for the simplest of apps.
Check out the NastyFans iOS App here:
https://bitcointalk.org/index.php?topic=428019.0PM me with more information if you have funds and are serious.