The more developer time you spend planning and designing your app, the more it will cost. You can save yourself money by doing a lot of the initial concept and planning yourself... It doesn't take a design degree or much technical know-how; just a pencil, some paper, and a little common sense.
Most time in planning and designing an app is spent ironing out the fine details; Small details that are often overlooked when discussing an idea in more general terms, but have to be addressed when it comes to implementing the idea technically.
For example, in general, you know you want your users to create an account before using your service. But what information specifically do you ask for? How many of these data fields are optional, and how many have to be filled in when registering? What constitutes valid data? Would a US postal code be acceptable, or should your users provide a valid UK postal code?
By putting this much thought into the behaviour and requirements of your app prior to meeting with a developer, you can dramatically save the time it takes to design the app, saving you money!
Think, in as much detail as you can, about the product, the screens, and how it will look. What information does your app require in order to work, and to give the best user experience, and where does this information come from?
Go over your sketches from Look and feel and imagine yourself actually using the app. Think of each case, not just the success cases. For example, what happens if the user enters invalid information?