My advice is to find a few apps on the app store that are similar to the one you would like to create. Next, go ahead and create your app using 3x5 notecards. Put your screen view on one side and a description on the other. At that point, I think you are ready to find a developer who can give you an accurate bid on your project. If you have a really cool idea and/or are providing content, some developers might even split the development costs and revenue with you.