Take my post with a grain of salt as I don't have any real experience (except from working and researching on my own project currently), but from what I've read on here, $4k-15k will get you an excellent app developed. But then there's marketing costs to bring in sales, and website development costs.
Just make sure you have the legalities all taken care of as well. First thought that comes to mind is that the developer needs to sign an employment agreement that anything they produce is owned by the company.
I'm no lawyer though, but just something to do research on in case you haven't already.