Hi, I have gone through the Adobe DPS process for my own publication, there are many decisions that you need to make prior to going digital and this will all impact the cost. I think the first phase of the project should really be some discovery/consultancy to help you make the required decisions.
There are also alternatives to DPS, The cost I estimated is more of a guestimate because the cost will depend on many factors like the total number of pages/interactivity elements/horizontal or vertical or both, retina display or not, IOS or Anrdoid or both.
PM me if you want to talk about it ... I can discuss the project with you during this week, but cannot start any work before 5th July