create book ipad / iphone app ( iOs )

Hi, in a few months we will start a new book project. The book will be about 250 pages (text) and will contain about 30 pictures/illustrations. We are experienced graphic designers ourselfs and we will create the book in InDesign for print and as an epub file for online digital book stores. But, this time we also want to explore the possibility of creating an App for the iPad/iPhone. For this we are looking for help in developing and bringing it to the Apple App Store. The App doesn't have to be very fancy, it can just work like and have the same functions as an epub. We would like to know what the cost will be for this. As said, it will take a few months before we begin this project, but we are investigating the costs.