There are many prerequisites which you must be clear about to estimate the expenses of the app. Here are some questions which you can ask yourself to be clear about your need:
What do you want the app to do?
What is the expected traffic flow?
What is your marketing strategy?
What do you want the customer to do on the app?
What is the purpose or business aim that the organization wishes to achieve through the app?
Once you are clear with the above questions, you can contact different agencies about the app. Every agency is going to quote a different price. So, be ready with it.